Have you spoken with the folks at Milltronics yet? Their online support has been really first-class.
You have one control upgrad before you go whole hog on a brand new controller.
They still support the Cent V -B (that's dash B) serial number controllers and can install a single board computer along with a bunch more memory, for just under $2K. You need to remove and send in the control box from in the back of the machine. They test your drive cards, install the new computer, test and return it.
The serial number is on the underside of the control box and if you are calling in to tech support or emailing them, it is useful to have that and the machine serial numbers in hand.
You have a hard drive? Mine does not. SimDisk only. Vibration is tough on a HD. You are pretty much shopping used on ebay for an HD that you can use on this era computer.
Anyhoo... Check your serial numbers, call tech support, or email them. They were really helpful when I needed the help.
The Cent 8 WOULD be good, but maybe more money than the machine is worth spending upon, given that you either have to be able to ship it to them, or pay the bills on having a fella come there and install it, if that is even possible. IIRC the Cent 7 upgrade was on the order of $8K, I don't imagine it gets cheaper...
Never programmed with the conversational.