This machine, while still having potential, is getting old enough, you may or may not have a hard time finding some parts. My advice, and I think I speak for a good many on here, find out where it is located and have the local Haas Factory Outlet send out a technician to get a non-biased opinion on the condition of the machine. It will be a few hundred dollars that may save you thousands.
As to your questions:
The Haas control is NOT PC based. It is proprietary. Any parts you need will have to come from Haas, unless you are lucky enough to find some used parts on ebay or from parts machines.
Few machine parts are cheap. Especially from the manufacturer. We had a drive go out on our Citizen swiss machine last year. There were none of that type available. Our options were to spend $12,000 for the retrofit to a newer drive, or $2500, and 2 weeks down time to have the old one rebuilt.
Manuals are available. The operator manuals can be downloaded from Haas' website. The service manual you have to buy from Haas. I am not sure what their price is for it.
Electrical/control components, being proprietary, are not really user serviceable. In fact, the service techs are not really trained to know complete diagnostics. Their general policy is to find what is causing the problem, and replace it. So if it is a processor board, they don't diagnose the exact circuit, they just replace the whole board. Small things, to a certain extent, can be fixed. We fixed the green light on our beacon by replacing a bad resistor in the beacon. But not all fixes are that simple.
All, in all, you have to consider that you are looking at a 20 year old machine. If the tech checks it out and gives it the thumbs up, have fun! If he calls with a laundry list, I would look for another machine since it will likely cost more than you paid for the machine to get it fixed.