Sounds as if you are on a Desktop PC? not a laptop (you have "data device" Drive K).
On that basis - all is not lost - or should not be.
You say buy a drive and install W10 - yes that's the key. Then your old C: disk will almost certainly be visible post your new build (make sure it's disconnected in the meantime). It's "only" problem is that it can't boot - unlikely to be scrambled.
Or if it's a laptop - put the drive in an external USB enclosure, only £10 or so.
Outlook? Depends a bit on POP or IMap connection., and your use of any Personal Folders. But in either case your latest mail should be available in your User folder (either Documents or in Appdata) on ex C:, or even on your K: Depends how you set it up, and Outlook generation..
Old software? Yes, these often have data buried somewhere obscure on C: - but again your projects at least should be available. (Or on your K:?)
But installing old stuff could be a pain. In the 80s/90s I started the a habit of keeping all old installation stuff, ex floppies, DVD or whatever in an archive (E,g, Corel Draw V3 1992, V7 1998, still in use 5 or 6 PCs later!) But if you didn't, could be hard work. License keys? look hard on your old C: after new build. a good chance they're hidden there somewhere. Or previous downloads may still be there?
Good luck.
And - to everybody else here at or approaching Thinkdeeply's predicament - back it all up to some external drive, now: And "all" means lots of nooks & crannies on C:, as above - NOT a W10 "backup" - it isn't what it says on its tin! And also use a tool like Cloneziilla or similar to clone on USB stick a precise & complete replica of your whole C: (Clonezillas is a bit Unix techy-ish intimidating, but works - other cloners are available, may be better interface.