Lots of good tips. When looking at an EM to purchase take a close look inside
the head and in the cabinet. The main thing to watch out for are signs of water
or moisture. It'll often show up as rusted parts and swollen wood that will
show up as gaps in corners. Look for obvious things like missing/broken parts and
cut wires hanging loose. Small things like missing pop bumper caps of PF
plastics can be hard to find. Most other parts are common to many other machines and easy
to find.
Cosmetics are important in the long term when you decide to sell. Flaking ink
on the back glass and worn spots on the playfield. Look long and hard at these.
Quite often an older machine will be filthy due to poor storage but will clean
up nicely. I've had that experience a number of times.
The best general advice I can offer is only buy a machine you can inspect
in person.
Hope you find a great project machine. Beware that this is a VERY addictive
(but fun) hobby.
Steve