Taking on an EM project is a brilliant thing to do if you fancy it. I am about two years in to a project to restore a non-working and pretty trashed Gottlieb Cow Poke and am close to completing it.
Its been lots of learning and very time consuming, but I've loved most of it. There are plenty of resources on the internet to help you with everything from reading schematics to cleaning and adjusting switches, which you'll have to do a lot of, and I have also had a lot of help and advice from the ever-patient
@DAD. Andy N. also paid a visit the other day to get to the bottom of one problem that I just couldn't fathom (and did so in about 15 minutes after I had spent about 3 weeks fannying around with it, annoyingly!)
I get the impression (although I may be wrong) that schematics, repro plastics, and general parts are much easier to get for Gottlieb machines than Williams EMs. Certainly Andy has some bits, and there's also pinballcenter.eu in Europe who have a lot of Gottlieb parts, or else Pinball Resource in the States.
If this is your first machine then it might be a good idea to get a single player one, maybe one from the '60s. They are much less complex and therefore easier to get going.
Most EMs have relatively simple paint designs on them and you can buy templates for them or I guess make your own if you want to do up the cabs. Playfields tend to be pretty worn, often to the bare wood in parts, but restoring them is not a problem if you know what you are doing. Or get someone else to touch it up.
I've seen a few for sale in antique/junk shops where they are nearly always described as vintage pinball machines and have ludicrous prices - £1500 etc. In fact you should be able to pick up a working machine for less than £500, and a non worker for a £250 or so. Finding them is the tricky bit, but other pinball people are a good source, or just ones that are in people's garages and they want to get rid of them if you can find them advertised in the usual places. Or Europe seems to be a good place to get them - I was after a particular cycling themed EM (a Recel Criterium 75) and after searching in the UK for ages I asked Jonathan Joosten from Pinball Magazine and he said he knew two people who had them in storage in the Netherlands who would sell them to me for £300. I found my Cow Poke by looking for one on Pinball Owners and found one for sale immediately.
) so I'd say go for it mate 
I'm more of a Bally EM fan with Fireball and Wizard currently in the collection but Grand Prix is a cracking game and I'd love an Atlantis at some point. The one I'd really like is a game called Underwater by Recel of Spain, that thing makes cool noises 