Ubuntu is a word in many African dialects that means "idiot who can't install Debian"
Also, Mint is nicer - you may prefer it to bog standard Ubuntu....
That said, been a long time since I've built a computer. Mobo choice will dictate what type of CPU and RAM (and how much of it) you end up running...
I'd start with Tom's Hardware "best build for $x" series -
Best PC Builds of 2017 - Build a Gaming PC on Any Budget