My advice would be to start with an EM from the 70's , especially a Williams EM as they play faster and tend to be cheaper (most collectors are obsessed with Gottlieb EM's) .
Also, if possible, try and find a machine that has the schematic and manual included .
Armed with both of those you can fidn and fix any faults .
If it doesn't have them , they are available to buy from places such as The Pinball Resource in the US.

I say choose a 70's game only because prior to about 1966 , far as I know, games only came out with a schematic , which makes fault finding harder.
A manual will have a grid reference to the schematic for every switch and coil , as well as explaining how things work .

Good luck in buying that 1st pin.