A couple things.
As for the control software, it depends how old they are. Just about anything 10 years old or older will be running a DOS based controller, if it uses a PC. And the PC will probably be 200Mhz or less (Mhz, not Ghz ). If the machine has multiple drilling heads, changing the control to something more modern may be very tricky to do, as a lot of machines use proprietary codes to control them. And alot of this control is done by PLC's, which may be hard to interface with (if possible at all). If it's a basic router, than the easiest way would be to rip out all the electronics and start over with new hardware that works with the control your using. Depending on your electronic skills and software choice, this can run from anywhere between $1000 to about $15-20,000.
If it's a machine with drilling heads, and it runs, I'd leave it as it is. If it's just a router, I'd try it as is and only upgrade if it makes the machine easier to use.
Parts will probably be very expensive, or impossible to get. Call the manufacturer and check on parts and service prices and availability.
If it were me, unless it looks in nearly new condition, or the price is to good to pass up, I'd stay away from anything not running or more than about 8 years old. Unless you know exactly what's wrong and what it will take to fix it. Just my opinion.