The way radio sizes are standardized you'll want a DIN size. I would say that because of the acoustics in the roadsters cockpit a radio and speakers with outstanding reproduction isn't as important as one that is loud. I doubt if you'll be able to pick up on all the subtle sounds that an elaborate system is capable of delivering.... especially when you're driving.
Pioneer, Kenwood, Sony or one of the radios that have a more classic look with modern features. A hundred dollars should get you a nice powerful modern looking unit. More for the retro's