I have no experience with networks, so I am a little bit confused.
From a big picture perspective, I would say hire a local contractor to help you with your setup and decision-making as far as your network configuration is concerned. A badly-configured network can be costly in both real dollars and opportunity cost if it was unreliable and or compromised; both are a real possibility if not setup correctly.
Technically, there's usually few reasons why you'd want to host your own Web server in-house; why not get a micro instance from Amazon EC2 or even go with shared hosting for now? Let somebody else handle the infrastructure while you can focus on growing your business.
And actually, why bring mail in-house? Google Apps for your Domain is free for up to 5 users and supports open protocols like IMAP, POP3 and a very good Web interface (just like gmail if you're not familiar with it) so down the road if you wanted to setup an in-house mail server you could.
If you want the Microsoft stack, Office Live 365 is a cloud-based solution that gives you all the collaboration/Unified Communications of Office and Exchange, but at a low monthly fee.
If you truly must have an in-house Web and mail server, consider setting up a DMZ interface on your firewall with filtering so that your mail and Web server are not on the same network as your LAN*. You can add rules to permit access to the DMZ from the LAN (but not vice-versa), but by isolating your LAN from your exposed Internet-facing servers, you're protecting yourself in case they get compromised. You don't need another static IP either: you can use DNAT (port forwarding) to expose certain ports on your servers to the outside world; this is a very common setup for small deployments.
*this gets tricky when you have an Active Directory domain setup and only one Exchange Server.
