E30 is the lightest so you'll likely get the most bang for your buck there. Problem with E30s is you lose the brake booster and the biggest tire you can run in the back is a 235/245. These are getting more and more expensive even for a shell.
E36 has a little more room in the bay and for the rear tire. It will come to your personal opinion of E36s - I personally don't like the aesthetically so I would skip the E36.
E46 is a sweet spot in my opinion. The electronics are a little more advanced, but not so much where you need to spend thousands to get your gauges working. Plenty of room under the hood. Plenty of room for a big tire. Really great aftermarket support for other parts.
Anything past here gets cool, but has less aftermarket support or is more expensive. E87/E92 would be rad, but again, more expensive.
The E39/E34/7 series is a different animal. Are you looking for a sports car or a cruiser?
As for the E31 argument the V12 in that car is a joke. I sold a manual 850i earlier this year and haven't missed it a day. That would make a cool LS swap as an overall improvement to the chassis.