Q1: Look at your current coin comparitor and retrieve the info off of the sticker on front. It will tell you model number (CC-xx) and also the voltage (12v or 24v). Replace with IDENTICAL model and voltage unit only!
Q2: You will need to insure the software of the new validator is compatible with Bally machines by verifying the protocol ID. Bally requires an ID003. I believe Sigma uses same protocol.
Q3: Without changing housings for the bill validator, you can use a DBV200. This is an older validator as well, and will only take bills up to the colored $5 (I believe, possibly the colored $10). Unless you change entire unit (housing, harnesses, transport, and head) to a WBA model, you will never be able to accept newer bills. The manufacturer (JCM) discontinued support for the DBV models some time ago, therefore, no new software updates will ever be available. Cost of complete changeover to WBA would be approx $100-125. The WBA will currently accept the colored $100 bills.