Advice on bringing email and website in-house

We're a small architects practice (5-10 people) planning to bring our email and website in-house. We'll be running Lion Server on a new Power Mac and we'll have a minimum downstream connection of 16 Mbps and an upstream connection of up to 10 Mbps.

We'll be running most other services on the same server, including AFP, DNS, iChat, Wiki, iCal, Profile Manager, VPN, SMB, Address Book, Open Directory.

Does anybody have any advice they'd like to impart? What sort of redundancy should we aiming for? Should we have a Mac Mini on standby in case the server blows up? Or is it possible to have an external email/website hosting service on standby (how quickly can we switch in the event the server goes down)? And an additional internet connection with a different ISP I imagine is a must.

Will this setup be sufficient to share files directly with clients (mostly nothing over 100 MB)?

What about security? Is it unwise to run services like email and web on the same server hardware as our wiki and file server? We're not dealing with high-security data, it's more a case of not wanting to deal with any downtime that security holes might impose. By the same token I'm not an IT expert so we want the setup to be as simple as is practicable.