I have 3 A2100 powered lathes and they are great. I wish they still made them.
I also have 1 machine that is running Mach 3 and servos.
I would expect that if you complete the project, you would not consider it an update. Just my opinion.
Mach 3 is awesome. It is definately more versatile than most commercial controls, but it can't compete on the performance side yet. Hopefully in the future.
If you do gut the A2100 please send me an e-mail, email@example.com
I would be interested in the whole system, if the price is reasonable.