IIS and Exchange on same server with VPN

I am looking at the design of the network of one of our clients. I would like to implement VPN with them (they currently use dial-up for RAS). I would like to see if the following would be a good solution (at least in the short term):
Right now they have Exchange and IIS on two different servers. I would like to put both on one server and use the other as their file server. Which server would be best to put RAS on for the VPN access? And would placing a software firewall on this server be justtoo much for the machine to handle (they are very nice, new HP NetServers)? Thanks for your help!