It has been discussed many times on other boards. The major problems with designing your own pinball machine is that the playfield is alot more difficult to get exactly right so that it plays well. The angles are important, and the metal on the playfield is always custom for that machine. You will also spend a major amount of money on custom plastics.
The other problem is that a broken pinball with a good shape playfield can be fixed much more easily by getting replacement boards, than trying to wire 60 or so switches and at least 50 or 60 different lights into a computer. Remember each switch has to be individually scanned and processed, and each light has to be individually controlled. There are also medium voltage parts (solenoids), that need switching on and off. On a solid State machine, there is typically a CPU board, a power board, a sound board, and a light/switch board.
If the playing surface is in great shape, and the backglass is in good shape, the price will be high anyways, because a new control board (or a fixed replacement one) can be had for a good price. For example I have a Space Shuttle machine, that had a bad CPU board. I bought a completely fixed one from a great person for $125.00 For the Bally machines like Playboy, and Space Invaders, there is a replacement board available from a company called Repair connection that is around $300.00 On the Bally machines the sound board can be had for a good price, and the light control boards are very inexpensive. There are companies like pinball resource who sell pretty much all the parts you need for pinball repair.
Basically fixing a broken machine is much easier than trying to build a new one.