Firewall for WISP

1) I'm setting up a WISP and wondered what firewalls others are using for this type of network. I've been looking at the FortiGate 60 and wondered if it was overkill or not. It has a lot of features. Primarily, I want solid and reliable--don't need unhappy customers. It's going to plug right into a T1, so IDS is just important as well. AntiVirus and unlimited users are also nice. I would like to give the customer as much freedom as possible without compromising the network.

2) Are there any like this that also include a mail server?

If it matters, I'll probably be using MeshAP-Pro nodes for the network, and if the answer to #2 is no, I'll be using w2k3 Enterprise Server for pop3 and smtp.

Thanks