On 30 to 40 year old games, I think where you're starting from regarding the machines condition is way more relevant than outright maintenance.

If you've played them all and can only have one machine, pick the one you like to play the most. Thats way more important.

TOK is right about gameplay, but as far as your specific question on maintenance goes, I would recommend the Bally solid state games, Star Trek and Eight ball. There's less moving parts on solid state games. They made many titles in this era that use the same parts, and the production runs for these titles are huge. As far as running parts go, easy to find or substitute. They are also fairly easy to troubleshoot. There's also upgraded aftermarket replacement boards available.

I'm not sure if you have the date wrong or what, but the Eight Ball 1984 date is confusing me. You might check http://www.ipdb.org to be certain which game you are referring to.

Bally Eight Ball 1977
Bally Eight Ball Deluxe 1982
Bally Eight Ball Champ 1985

EMs will require more tinkering than SS games. However (my opinion) EMs are easier to troubleshoot because issues can usually be tracked down to a switch not closing.

But get whichever game you enjoy the most and learn to work on it.