whats the difference?

Between the SE's and the SE'R 200sx's besides the looks and the engine? I mean if the suspensions the same and the brakes/both have abs, wouldnt it make more sense to buy an SE and pop in a SR20DET? Im just wondering because SER's are impossible to find and everyone has SE's

Nope, the SE-R's have a limited-slip differential, four wheel disc brakes, and you need all the SE-R specific components to drop in that SR20DET. The 200sx SE has the 1.6 liter engine which means it has the 1.6 ECU, the 1.6 MAF, and the 1.6 tranny/axle. So it would be quite a bit harder to SR20 swap an SE. Or you could just get the SE and get the 1.6 liter turbo kit from Hotshot.