I wouldn't advise to spend any money in a system that old.
That GPU is not going to handle any game, no matter what CPU you throw at it. I have an HD5750 1GB, and while it can run games at 60fps I have to turn almost every setting to the minimum.
You need a new CPU, MB and GPU if you want to enjoy any game. And I'm pretty sure that DDR3 would be a good improvement in general performance, though I have not look at any kind of benchmark to back this.
This hasn't changed in Windows 10. Cortana come disabled by default, nobody is forcing you to use any program you don't want to. I really don't know where this fears come from....
Libraries where introduced with Windows Vista, and since then you can choose where they are. Mine have always been in another HHD, just the same as when I used Windows XP.
And after the upgrade, you obviously don't lose any setting (except for default programs as Windows 10 link some extensions to its new apps).
As for your other questions, I am not 100% sure on them.
You can install Windows 10 with an offline profile, but it may pressure you to use the online account.
Windows 8 was the best Windows ever built, with just one major flaw in its design. Windows 10 runs circles around any other Windows 10 ever.
Do a backup of whole LFS' folder, after doing a clean install restore the files inside "misc" folder, it should keep the game unlocked allowing you to play.
Worst case scenario, you have to restore the folder you backed up.
Not really. The pixels in the skin are mapped to a 3D model that you see on the screen. Skin resolution is probably more related to screen size than screen resolution. A 4096px skin could be low res even in a smartphone screen, you just need to zoom enough
In my opinion 4096px skins are not worth the space they take on hard drives (wether it is mine or at LFS' HQ) compared to the visual improvement. At least not in 2015.
I guess that the current plan is to have 1024px free and 2048px as paid service? Or is that option going to be revoked?
Once you are using that adapter, Windows will see the wheel (wheel+pedals+shifter) and another different USB device. You should only worry about games that only allow you to use one device (luckily there are less and less of these badly designed games nowadays).
Just use InSim.net if you are starting any project. LFS_External will be a pain in the ass if you wan't any sort of compatibility with newer tracks and InSim changes (40 cars, new layout objetcs...).