Sorry Steve, I had to go to work!
Well, considering your desire for a single location you could consider the following....
1. Stuttgart area - There's Schweickhardt in Stuttgart and Seyfried in Pforzheim - but there's also a museum in Goppingen run by that mob that make the 3-rail stuff. So if you are of that persuasion it could be a nice visit all-in-all. There may be other shops in Stuttgart too, but I don't know.
2. Nuremberg area - Ritzer on Bucherstrasse and Doerfler on Faerbestrasse are two good shops - but there is also the wonderful DB museum which would also make the trip very worthwhile. Everything close together, just a tram ride (#4 to 'Thon' from memory, get out at Juveneilstrasse) to get to Ritzer.
I admit that these would be my choices, there may be great places up North - indeed I have a few of the well known shops including Modellbahn Kramm at Hilden in 2011. But I do like down south a lot!
Happy to give beer advice too if you would like.
Cheers
Phil