BMW S14B23 or B25 if u wan to go NA route , these engine produce a awesome intake roar !,found on E30 M3 or M3EVO .
For turbo , SR20DET should be alright , it is reliable if u don't squeeze big boost into it .
U should stay within 4-cylinder engine as to keep the weight distribution near to 50-50 , on E30 with 4-Cylinder engine the engine should be mount just behind the front axle for good handling , thats why when BMW design the E30 M3 they use the 4-Cylinder S14 engine from thier F1 machine rather than using M88 or S38 inline-6 because they want to keep the weight distribution close to 50-50 for agile handling .