Either way it's fairly trivial, whether recommended or not, to config the TMG host to act as a DHCP host as well, just enable the server role and punch the appropriate holes in the firewall.
Do note that TMG was designed to function in a domain environment and if it's not added to the domain you may have to do a few things manually like importing certificates and such to get it to talk to the exchange server securely, either way it's pretty easy to setup
I loved our TMG. We ran it within EBS and once you got the hang of it everything was fine. We used it for NAT/Proxy/Firewall.
I'm debating about doing the same thing as another poster here, putting this in front of my exchange and maybe sharepoint farm. I don't think our Checkpoint device would do what TMG can do for these products.