1. Are the Viper 200s necessary or will the 95s be adequate?
Definitely the Viper 200's since the motors have a lot of Mass. You need a big drive to stop those since they can generate over 100 amps as a generator on decel.
2. Will they drive those old ElectroCraft DC servos properly?
Yes, many have used them
3. Do they have an interface to work with the tachometers on the drives?
3.b. Is the tachometer necessary?
No you don't use the tach signal at all.
4. What reasonabley priced encoders will be easy to setup and work with good resolution with the Viper drivers?
US digital. Use a differential encoder on an industrial machine (don't save $10 and get a single ended) 1000 count E5 or E6 model
5. What is a good breakout to use between the PC and the Viper drivers?
I have a new one or check other vendors.
6. Does the Viper handle correction with feedback from the encoders or does it count on the control software to do that? (That would eliminate Mach. I don't know about EMC.)
The Viper has the PID servo loop built in. Mach only tells it where to go using Step/Direction signals
7. Can the Viper controllers provide encoder data back to Mach for DRO like operation?
I wouldn't recommend using the same encoder for DRO. You will get noise and ground loop problems. Also Mach3 can't read a encoder fast enough to keep proper count. You need dedicated hardware counters to read an encoder. Mach just polls the encoder inputs.
8. What questions should I have asked, but didn't?
Hmm. You will think of a lot more when you go to install.
On the Hurco, the encoders may be ok, but i would run new wires to them (twisted pair). I had a customer where there was a connector hidden in the machine that caused problems.