I'll give you some perspective from a guy that had zero pins, and decided to start with a $50 Gottlieb EM I picked up on a whim. I followed that up with another $50 EM (Bally this time). Neither were working when I picked them up. There was a world of difference between them though. I don't know anything about non-EM pins, so I can't offer comparisons just yet.
Short Version: A clean, complete machine is your target. Working is a huge plus, because maintenance truly is minimal on them it seems. They were basically designed to run mechanically maintenance free to limit the number of service call outs, etc. If you want a project, then finding a clean, complete non-working EM is a great buy if you can find it cheaply. As others have said, sometimes it's as simple as a single switch contact or blade out of alignment. The hardest part about working on an EM is learning to read the schematics and tracking down each little issue methodically. The folks here on Pinside are awesome with this stuff, and I've found that they are more than willing to help. Trust them, give them the info they need to assist, and they'll guide you through your issues.
The Gottlieb I grabbed was complete, but it was rusted inside and out. Literally every single thing that could have rust on it did, most of it completely covered. That should have been a red flag right away, but knowing nothing about pins or EMs at the time, I figured what the hell, $50, I can decorate with it if I can't fix it. The playfield was in good shape, and the backglass only had flaking in the "red" painted areas, which is common. Mechanically, it was a bear to get working, but mostly because of all the rusting and sticky bits and pieces. In the end, a huge part of the repair was just cleaning, lubing where possible, checking alignments, etc. It was tedious, but not hard. Even more tedious has been the slow-going process of getting every little light working again, since basically all of them were so rusted they didn't make good contact, etc. I still have a couple dozen of those to go.
The Bally was the opposite: it was pristine inside. Every bit seemed shiny and practically looked new to me. It was missing a couple of coils and parts, but they were easily replaced. Getting it working again was mostly a matter of a few switch adjustments and contact cleaning, very minimal effort thanks to some of the great folks here on Pinside offering up assistance. The game plays great, and everyone enjoys it, which to me is the most important part at the moment.
Final bit of advice: unless you find a sought after title or more valuable EM, don't pay much for it even if it's working (unless you don't care about return on investment or money down the road). From what I've seen, there are a lot of EMs that simply aren't worth a ton of cash. Even the $50 ones I picked up weren't that big of a deal. Do a little research, or a price check on here before throwing down too much cash. If your area is anything like mine, EMs will sit on places like Craigslist for a while when they're overpriced. I regularly see them get listed for $800, only to drop down to $200-$300 range after a couple of months. Just be patient, you'll get a good deal.
