I agree stay away from 4 player units till you get the basics of a single player unit. Most of the time you can fix a machine with cleaning and adjustments. If you can get the schematic with the machine your ahead of the game. 90 percent of games you end up rebuilding the flippers. all games you end up buying all new rubber. NO contact cleaner use alcohol, No grease on anything but what manufacturer specified. If you get a machine where some idiot has used grease all over the place you will end up cleaning everything to get it to work. Get a spring kit off ebay they are cheap and when in a pinch for a spring they are essential to have around. Get a real contact bending tool cheap but essential especially when adjusting AX and BX type relays

I think the weirdest problem I ever had was on my first machine. 500 point rollover switch would cause the score motor to go in perpetual motion. Eventually got around to checking the voltage on the 500 pt relay and there was nothing there yet the relay was still engaged causing the score motor to spin. being that it was 40 years old the armature had become magnetized which was causing the problem. took the relay apart and took the armature to the buffer hoping that the static electricity would demagnetize it. It worked ....problem solved. Hul1gn is right after you fix it , it will be highly rewarding, for a second you will feel like Wily E Coyote "Super Genius"