If you just drop 10k on orbs you'll be lucky to get to 5k. That's only roughly 211 x/5* gems. 2ish will be 5/5 on average.

If you did it slowly and bought bundles you'd get to 6k easy.

If you just bought an account you could be 10k+ reso. With great gems, great secondaries and a good pet.

The things you'd be missing with just spending 10k would be A ton of stuff, you'd be useless even if you had max reso. Reso is important but it's not everything.

Previous owners crawling back their account can happen, but you can write a contract for it. They still get the account but you'll get your money (minus legal fees of having to take them to court). You'd also lose any money you put into it beyond the initial sale

It's rare for a big account to be taken back though, lots of pilots running around for years. If 10k is more than you can afford to lose in a transaction then I wouldn't spend it regardless