Whats the model # of the largest motors you have laying around?
The noise would depend on your drive method, if its direct, what kind of coupling. If its divided down, what type of belt. I see no reason if using a v-belt it would be any louder than a stock taig, as long as the RC motor has decent bearings.
Some related threads but I didn't see any answers to your questions:
Can RC Brushless Motors be used as Spindle Motor ???
Sherline spindle motor suggestions?
High Speed Spindle for SIEG X2 Mill - getting technical