Quick few thoughts.
Gottlieb have most part availability and best quality of parts.
Can buy so much here -
Can also get schematics from them.
Single players much easier to work on - multiplayers have extra reels and complicated player units!
New backglasses when available are expensive, so always try and find games with nice backglass and playfield as the rest can be restored.
As for titles - so many. 70's Gottlieb drop target games are very good.
Top 100 on pinside just for a starting point.
The Pinside.com EM Top 100 - The best EM pinball machines, as ranked by thousands of pinball enthusiasts around the world!
pinside.com
I have done many 100's of hours on 50's Gottliebs - when they are finished and all faults sorted, no reason these games won't be running in another 50 years!
Talk about over engineered. David Gottlieb said if he had known how long they were going to last, they would have used cheaper parts but his company motto was 'there is no substitute for quality'.
Oh, and take your time buying the first - very easy to pull trigger on first game that shows up.