Aracil,
Windows doesn't do spanned drives on the boot disk well and with Windows10, MS removed quite a few features, that's probably one of them. It's also a bad idea to do for data safety reasons, it's a step above RAID0, but not by much. You can remap your user directories to a second drive without spanning, which is a better option as it leaves data intact and you always know what drive that data is stored on.
Fohat,
My understanding is you can re-use your old key and it will take it, but I don't put a lot of faith in that method. If it fails, you're kind of out of luck without calling them and talking to someone, something I despise. It's a computer, why are we having a human intervene? Just seems stupid. What you want to do is look online for Windows key code extractors which will tell you the actual code you are using. This makes it far more safe as you have the exact windows key code. Write it down, keep it somewhere safe.
That said, when I setup used systems for people I try to use Win8.1 when I can, though some do go for Win10. You can still use Win7 or 8.0, however support for them is already started to slide, manufacturers simply want it gone because it means less drivers to support. Besides being newer, Windows 8.1 uses the same driver system as Win10 so there will be longer ongoing support for it. Personally, I prefer the Win10 interface and underlying system, however Win10 brings along so many privacy and update issues I simply refuse to recommend it so long as 8.1 is a viable option. Ignore the "WIN7 forever" crowd, people did the same with XP. You can't even (easily) install Win7 (or 8 for that matter) on a new Intel or AMD system.