The E30 chassis has a decent suspension and the rear trailing arm configuration was used in all E30 models as well as some newer cars including the E36 318ti and the Z3s. These later models give us options of fitting the E36 rear suspension from a 318ti to the E30. I have heard of people trying to fit the Z3 rear end, particularly the MZ3 or M Coupe would be of interest due to the reinforcements, but I have yet to see a successful implementation. Treehouse Racing sells all the parts as a bolt-on kit. I have included a few pics of the E30 suspension below. A nice upgrade if your steering rack is leaking to to put in an E36 rack which has shorter lock-to-lock ratio. The E36 M3 rack is the shortest.
E30 Front Suspension