E36 chassis definitely, get a road car if you can and work on it! Most of the track versions are kept till they blow so be very cautious of buying one!

325i best of everything, 320i if you have no fear! Needs a lot of commitment! Get cut hubs off the McCarthys, spacers on the rack, welded 4.4 diff and away you go! Every engine in the e36 family will bolt up to your running gear so you can go all the way to M3 very easily.