If you're in Stuttgart you should visit nimpex and also the Porsche Museum!
Automobilminiaturen is good too and the owner is a nice guy but it's a very small shop ( basically just a room in his house ) with not too much stock. If you get in the area you can give it a try nevertheless.
If you're into 43rd scale stuff maybe this one could be interesting:
http://www.m-b-s-k.de/
Another 150km from the Ring but might be worth a visit.
Oh and since you're in Heidelberg the Technik Museum in Sinsheim is a must:
http://www.technik-museum.de/uk/sinsheim/