Hrm… that's odd. I was able to download them recently, but I don't doubt that you've had those problems.
However, I believe that the installers can be used even if they were downloaded by someone else.
I have all of the installers since Lion, and would be happy to make them available to you if you'd like. Send me a direct message with your email address and I can send you a link (the installer app is zipping right now).
I rarely make recommendations, but in combination with the excellent CCC, the Samsung SSD T3 (now T5) external drive is outstanding in terms of speed, size, weight and reliability ... if you can find it at a good price.
You can get 1TB SSDs at a good price ~$150, but 2TB (the size of my iMac-27) are still very high.
To @Jim's point, for a drive you only need for backup and emergency boot, this 2TB drive looks very attractive for only $64:
WD 2TB Elements Portable External Hard Drive - USB 3.0
But I'm not buying anything now until Black Friday / Cyber Monday.
I have not noticed any issues with KM after upgrading to Mojave, wouldn't recommend to update to Mojave as the system is new and no one knows when to expect any kind of issues.
Any machine connected to the internet should have at least the latest version of the previous version of the current OS. If you need to run an OS version more that one revision behind the current version, put it on a portable computer that you can keep off the internet, relying on sneakernet to retrieve data for it. Currently, I have High Sierra on a 2010 MacBook Air and MacOS 10.2.8 on a 2003 17-inch PowerBook. Both are disconnected from the internet, they use USB thumb drives for sneakernet -- and both run just fine.
I'm not sure I understand the need/rationale for this.
I have often ran a macOS two major versions behind the current, including right now: I'm still running Sierra: macOS 10.12.6
I've never had an issue with this, and continue to receive minor (dot) updates to Sierra.
Haven’t read all the posts here, but in my experience:
Sierra --> High Sierra: Immediately, without a doubt. Brings waste improvements to the iCloud sync, in case you are using it, and removes quite some annoyances from Sierra. Some folks are reporting issues on the AppleScript level, though.
High Sierra --> Mojave: Well, don’t know. I updated 3 weeks after the release, and I don’t notice any deteriorations since then. If you like Dark Mode, it’s a no-brainer, likely. Otherwise, I think the major improvement is the update of APFS, which now works with Fusion Drives, and all external hard disks, i.e. non-SSDs. (Maybe the latter has been true also before, not sure, but I converted my external HDs only now (with Mojave) to APFS.)
PS:
As a rule of thumb:
On macOS I would always update to the current OS version (not to the beta). One reason for that are the security patches, which are first and foremost applied to the current version. (Usually, if the thing is important, it is also applied to legacy OS versions. But I wouldn’t swear on it.)
But, before updating, check the Mac user forums, the user forums of specific apps (like this one), and the developer forum. Especially in the latter one you’ll find hints, if an update poses major problems.
Always up to you to judge if the potential issues outweigh the benefits. Usually they don’t, and issues tend to be hyped.
If still in doubt: update to the current version (not the beta).
My experience is that legacy macOS always get security updates. How far back that goes I'm not sure, but it is at least two versions. AFAIK, Apple considers all security issues important, and always updates the relevant versions.
Here's a recent example where multiple OS were updated at the same time:
macOS Sierra 10.12.6, macOS High Sierra 10.13.6, macOS Mojave 10.14 30
Oct 2018
thank you for your comment.
In a nutshell, what, in your opinion, and the risks and benefits of converting to APFS?
Well,
installers are not hard to find, but since I upgraded to Mojave, my system messages me about this or that "Not being updated for Mojave and future upgrades", so I have to inent something all the time. With the upgrade I got a downgrade file to High Sierra, which also had problems with some apps malfunctioning, especially when everybody is busy with updating their apps for Mojave, it seems like so one is going to work on their earlier app versions. So I am thinking about going back, but what's the use if half of my apps are already updated for Mojave?
Well, HFS is a very old (33 years old) file system. It dates back to 1985, and has seen a major revision in 1998 (HFS+).
This is a very good article (2016) from the point of view of a ZFS developer (for some time it was rumored that Apple will adopt ZFS as an HFS successor):
The article also explains shortly why a HFS successor was overdue, but above all it describes in a pretty readable way the advantages of APFS (and it critically also points out the shortcomings, as least as of 2016).
IMO, one of the major improvements of APFS is the Snapshot feature.
There is already backup software (for example ChronoSync 4.9, which I highly recommend) that makes use of Snapshots.
This should help for example to create reliable boot clones of the start volume: At the start of the sync progress the program makes a snapshot of the source volume, and then when anything changes on the source disk during the sync process, the sync will be done against the file state as per the snapshot.
It seems also CarbonCopyCloner, another good tool, is making use of snapshots. I’m sure in the next 12 months or so we will also see a new Time Machine that makes use of APFS snapshots in some way. (The old/present Time Machine with its thousands of hard linked files and folders becomes a maintenance nightmare, once the TM backup grows fairly large.)
Another big advantage is the improved reliability, well, at least I hope so. Things like the regularly due repairs of the B-Tree should be a thing of the past. (Until recently I had APFS only on the internal SSD of my MBP, for about 16 months, and so far I haven’t seen Disk Utility doing a single repair. I still have to see how the APFS works out on my external hard disks.)
Finally, a big point seems to be the encryption, which is now (APFS) a file-system-native encryption.
Another at least interesting thing is the file Clone feature. It’s not a full deduplication system, from a user perspective it works more like hard links. (But it’s not the same.)
But you really should read the article I’ve linked above…
Concering the “risks of conversion” (which was part of your question), I can’t tell you much. The conversion of four external hard disks was without any problems, but since it’s only two weeks or so that they are running with APFS, that doesn’t mean much.
Later this week I will convert the Fusion Drive of my older Mac Mini, and if I run into any problems, I will let you know.
Just noticed that the Mac Mini already runs on APFS since the Mojave update. Stupid me! See post below.
thank you very much for your comments, Tom.
The reference is very interesting and answers my questions.
Hello @Tom & @ronald,
after the release I installed macOS Mojave on my iMac with Fusion Drive and had no problems at all. 3 weeks ago I did a clean install on my iMac. Furthermore no problems with me.
Thanks for the report. So, maybe I’m gonna do it this night already 
@appleianer, I just re-read your statement: you just installed Mojave or you also converted the Fusion Drive to APFS? (I think it doesn’t get converted automatically when you install Mojave, right?)
To answer my own question: I just checked the Mac Mini (it’s used by my father, in another flat) and the start volume is already APFS! So, yes, it seems it was automatically converted when installing Mojave, 2 weeks ago.
And, here too, no problems so far.
Sorry, yes, I forgot to post a video, showing how I run to the flat of my father, booting the Mac Mini (he often shuts her down), and then showing the Disk Utility screen 
I know, you love videos…
Exactly... don't take my videos from me 
thank you.
