Good luck trying to "maintain" an E30 for $500/year..You may have to look at other manufacturers if that's the case. With E30's you have to be willing to roll up your sleeves and tackle some jobs or else it because a very expensive car to own. Once you take care of the things that need attention however an E30 can become a very nice, nimble enjoyable vehicle. You will have to put some $$$$ into it to get it to that point however.
As for E36s well they're newer, but not that new..The newest E36s rolled off the assembly lines a hair over 10 years ago so there will be things that need attention as well since some models will have a decent amount of mileage. If you can swing an okay E36 saftied and inspected by a reputable shop that specializes in BMW's and everything checks out, then maybe you'll be able to spend $500/year tops on maintenance.
E30 = More of an Enthusiasts/"project" car.
E36 = More of an turn key and go, Comfy car.