don't bother with an M20b20 (E30 320i) unless you already have one...
It will have less power than a 325 for the same weight..also will have drum brakes on the rears and smaller size front suspension legs...(unless it's a touring which has Mtech suspension and rear discs)
If you don't want or can't find a decent 325, a 318is (M42 engine) is a reasonable alternative...
The E30 325 has 170bhp and weighs 1250kg
The E36 325 has 190bhp and weighs 1500kg
and as standard the E36 328 is about the same power and weight as the E36 325
So power to weight is better in the E30...it also has a better balance and is easier to tune the suspension as well as being cheaper in parts, generally...
However, a standard E36 has a better suspension set up than the E30...especially the rear.