my 2 cents,,,,,
I'd say if you can get it for $100 or less that's a good deal, as long as the cabinet metal isn't total trash and rusted from like being in a flood. A machine that has been in a flood will often have many problems with electrical cable connectors, wiring, wire terminals, ground attachments that are damaged. This can be a major problem. But even if you never get it working the good parts from it could likely be sold for more than $100.
Although not as popular as other brands (IGT, Bally) the Williams machines are owned by many people so there is a need for good used parts. There are several Williams dotmation machine owners on NLG. As far as getting it working you can still find parts to repair it but they are getting harder to locate and prices for the parts have gone up. The dot matrix screen and good working cpu boards can be costly. Prices always vary depending on the seller. The missing cpu board may have been sold after the machine stopped working for another reason (bad power supply is common).
The dotmation machines have 2 power supplies and both are the weakest part of the machine and what usually causes a machine to stop working. Luckily they can be replaced with aftermarket power supplies that are low cost and more reliable than the original power supplies were. The lower power supply is for the entire machine except the dot matrix display and its associated circuit boards. The 2nd power supply is located inside the dot matrix metal box enclosure in the top part of the machine. This smaller power supply makes the 12 volts for the dot matrix display and dot matrix controller board.