1. Win 10. Anything older is a useless POS.
Ooh ... Very subjective. :-)
Personally(!) I've far far more problems on Win10 than Win7.
This is also very subjective: Personally I abhor the interface, though it's perhaps better than Win8/8.1.
For the record I have and still occasionally use Win2k, Win7 (twice: once dual-boot, once in a VirtualBox), Win8.1 (updated from Win8) and Win10. I have eight PCs, all running Linux as their primary OS, mostly Ubuntu, but my music-studio PC runs AVLinux (only).
Having revisited BIAB(2013!), with a larger screen, I'm feeling more comfortable with it and might consider getting a recent copy, but I really want to run it on that studio PC, though I might just accept the 10" notepad if I can make it useable enough.
_If_ BIAB will run reliably as a VST on AVLinux, I'd seriously consider that new version, but as it stands, if I have to get a new laptop, with it's own Win10 just for BIAB, particularly as I'd want to use it mostly via a Focusrite Saffire so it would also need Firewire or Thunderbolt, I really can't see me being prepared to part with the cash that would entail.