I think the easiest option would be to just get a USB controller, like a cheap USB joypad - then hack it to provide button input for the game, and coin detection using a standard cabinet coin slot. The cabinet coin slot is probably quite expensive, but not bad if you consider how much these machines can make.
In DBPro, you would just check the joystick buttons for action - the coin slot may be a bit tricker, and involve some trial and error.
The fancy, proper option is to use a keyboard controller board, which would connect via USB or PS2, and just have a bank of points that cables attach to, so you'd have everything wired up properly that way, plugs going to microswitched buttons, and to the coin door. The major benefit in using a controller board is that they have much higher bandwidth than normal keyboards. If you use a normal keyboard, you can only press 3 or 4 buttons at once and have them register, but on a controller board it's more like 12 - makes a big difference with MAME cabinets, esp fighting games. Imagine only being able to recognize 1 or 2 buttons at a time on a StreetFighter game for example. It might be fine to hack a USB joypad or keyboard for a simple gambling game, but coin doors might not like it.
I am the one who knocks...