EM pins are simple and straightforward to work on, and almost always involve repairs requiring very inexpensive, simple parts. All you'll need are a couple of screwdrivers, a meter, and some other basic tools. Just make sure that all the parts are there and that the playfield isn't worn through and you should be good.
Solid State pins can be a mixed bag. Look out for battery acid damaged CPU boards. Look inside the head and be sure that ALL the boards you need are there - you should be able to see empty spots where boards were *supposed* to go. Missing boards are a real possibility on SS pins, since the same board works in multiple machines. Also be on the lookout for missing displays, cracked displays, etc - some of those displays are very expensive. The biggest pain in the neck with SS pins is that you run the risk of needing some expensive part - display, board, etc. So watch out. Plan on replacing connectors - most solid state pins are known for crappy connectors, so expect to see burned ones. Burned spots on boards are also pretty common, some parts run hot. Inspect to make sure it's something simple like a hot running resistor or diode, and not an exploded chip....
In all pins, don't worry about degraded rubber parts - you can get a kit of all the rubber rings and bits you need for like ten bucks from Pinball Resource. In any pin you get, unless it's fairly new or been maintained, count on replacing all the rubber parts, the pinball itself (gets worn and pitted), and a whole pile of bulbs. These parts are all very cheap - around $20 to do an entire machine.
In general, look for the same kinds of things you would on video games - evidence of moisture, rust, corrosion, dead animals... Look for backglass flaking and playfield wear. Make sure all the plastic parts are there and not broken.
And, of course, look for machines that you like, or that you would enjoy.
-Ian