I've got a slew of TM500 equipment, and while some of it is non-functional, the power modules and plug-ins that are working have been pretty reliable. A PS503A is a nice little power supply, though you can get good stand-alone units pretty cheaply these days, so it's not a slam-dunk. With the exception of the calibration fixtures I have not had any trouble sourcing replacement parts for plug-ins or power modules. The most troublesome components have been the LED displays, and that's just because I've been trying to get original parts: I think that I could replace most of the 7-segment LED modules with modern parts without too much trouble.
The TM504 is a nice compromise between capability and size. It has one of the high power slots, which you probably want if you're going to use a power supply plug-in, and it's not nearly as ungainly as a TM506, which is an absolute monster (and would not fit under your cart, as you observed). I like the TM503, but it lacks a high power slot, so it's not the best choice to use a power supply in. It's very nice for working on malfunctioning plug-ins, however, and it makes a good home for my calibration fixtures.
-- Jeff Dutky
