The biggest problem you are going to run into is that most "real" VTC systems are impossible to just buy, but one is available through distribution. Do not go with an inexpensive solution. Life size is the lowest end I would go but only if a client wouldn't go for Cisco or Polycom. They are the industry leaders for a reason. The clients IT department is going to need to be kept in the loop the entire time. You're going to need to know how strict their security is.

To be honest you're client needs to be prepared for a $30k bill for a simple 2 cart type system. I would also recommend doing the yearly maintenance contract that the VTC company offers. It's a nice recurring revenue stream and it almost guarantees that the client continues to call you for all A/V needs.