Here's my stock:
Mainframe, 4 slot TM504
Mainframe, 4 slot TM504
Mainframe, 3 slot TM503
Mainframe, 1 slot TM501
Extender for TM500-Series 097-0645-02
Power Supply, Precision PS501-1
Power Supply, Dual PS503A
Generator, Pulse, 50MHz; dual Output PG507
Generator, Function, 40MHz FG504
Generator, Leveled Sine; 0.05...250MHz SG503
Counter/Timer, universal 125MHz 8Digit DC503A
Delay, Digital DD501
Latch, Digital DL502
Multimeter, Digital; 4.5Digit RMS DM501A
Amplifier, Operation AM501
Amplifier, Differential AM502
Amplifier, Current Probe AM503
An important thing to know is that Tek series 5000 plugins are totally well usable instead of the empty TM500 plugins - they are available at very low prices and you only need the PCB and a new panel.
Besides hunting for more plugins, I am now looking at building some special devices using this system.
For example, a fast power buffer to raise FG/PG outputs. I think that this is one of the main advantages of this system.