If you cang et some early model rx7 coilovers to fit, do that. Coilovers and welded diff will be a very good start.
It will be able to drift with a stock motor with diff and suspension done. You'll just need to enter the corners a bit faster

The rb20 and commo motor might be cheap, but they are also heavy. Just cause it fits, doesn't mean it's good. The weight balance (particularly given the relatively short wheel base) will be terrible.
I think if you want a cheap motor, do a ca18det conversion. Cheap, much lighter and effective.
Otherwise, if you can save some more money, do a s4 13bt swap

It may not be the "ideal" paltform, but you can sure as hell make it work!! The basics (suspension and diff) will be cheap too.