Look to make sure the monitor works if you plan on using the original monitor. They're usually fixable, but a pain in the butt (and possible dangerous) if you've never done it before. Also make sure the cabinet is JAMMA compatible. Most games made in the last 20 years are. It'll make wiring it up alot easier if you have a JAMMA cabinet and a J-PAC keyboard encoder.
Installing extra buttons is as easy drilling holes for them into the control panel, screwing them in, and wiring them up to the keyboard encoder. Just make sure you have room on the control panel for the buttons you want to add. Otherwise you'll be building a new control panel. If you're adding to a metal control panel, make sure you have a way to make those extra holes. Wooden control panels are always going to be easier, but most older machines used metal control panels.
The best way to transport a machine is standing up. However, I've laid machines down in the bed of a truck before without a problem. If you're going to lay it down, though, make sure anything loose on the inside has either been removed or secured. Things that stay in place while a machine is upright might fall when you turn it on it's back or side.