Hi,
The old 64s tend to work, the main failures are apparent pretty quickly, some things not obvious straight away are as follows:
test for sound (sometimes the chip has been swiped for other projects or is broken) and the serial /joystick ports can be dead and the machine will boot. Other than that they tend to keep on running ;-)
Will be passing on my Dtv to Chain to mod later this year and been playing with the 1541 III sd card drive and looking forward to the 1541 ultimate. Minimig will be fantastic if it can be cased up and integrated with a nice monitor as in the demos on youtube.
Ps well done on having a a4000, one commie I always wanted :-)