I did a similar bid (for a manufacturing and sales company with 40-50 employees) a while back.
Unless you plan on growing to over 75 employees in the near (less than 2 years) future, you could do the whole thing with as little as one Windows SBS 2003, Premium Edition, server. That'd give you a good firewall, an email system (Exchange/Outlook), SQL Server, and full remote access for all employees. But if you are running line-of-business applications, you'd probably want a second, Windows Server 2003, box just to run that application.
If you have the need to do heavy-duty web serving (few companies do, unless they specialize in Internet sales), then you might want a separate web server, or could host offsite, depending on what your web content is.
You'd set it up with a single inbound hardware firewall (if you wish...it's not mandatory). Plug the SBS 2003 Server into that. Then plug your switch(es) into the second NIC on the SBS Server. The built-in ISA 2004 firewall will monitor, filter, and control all incoming and outgoing traffic. It'll also provide VPN services for your company. SBS's Remote Web Workplace will provide full remote access to every PC in the company. The built-n Exchange 2003 SP2 will provide full remote access to email to SmartPhones, PDAs, and other remote clients.
Set up internal RAID 1 on your SBS Server for the OS. Use a second disk array of some ort (RAID 1, RAID 5, etc.) for data storage. Add a tape or removable drive system for offsite backups.
Management of a system this size, with 50 clients and a qualified person managing them, would probably take somebody about one day a week, mostly supporting the client PCs. The Server itself should only take a few hours a month. Most companies this size would use an outside consultant to manage the Server and the network, and train somebody inside for part-time support duties.
It'd also pay to install a client monitoring system, to monitor the health of the XP client PCs. An IT consultant can provide this monitoring service for about $150 a month.
Costs:
MAIN SERVER: SBS 2003 Premium Edition with 50 CALS: $5000
SBS Server hardware with two RAID arrays and 500GB of disk space: $5000
APPLICATION SERVER: Server 2003 Standard Edition (no CALS required, they are provided by SBS): $700
Server 2003 hardware: $2500
Spare Server and parts - I'd buy two basically identical boxes for the MAIN and APPLICATION Servers, and then buy a third, bare-bones, identical box for spare parts. You'll want a couple of spare hard drives, and a spare RAID controller card, too. - $2500
Hardware Firewall (optional) - $1500
Tape or removable drive backup systems: $2000-$5000
Switches and UPS: $2000
Server-based Antivirus/AntiSpyware and 50 desktop deployment licenses: $2000 plus $750 per year.
Line-of-Business applications - $5,000 and up
Labor - about 60 hours ASSUMING all client PCs already have Windows XP Professional installed and existing network cabling. This includes migration of old emails from POP accounts to Exchange Server.
Note that these specifications are based on "typical" needs. If you get a lot of email, host ecommerce sites, store large amounts of data, or have other atypical needs, the storage size, number of email servers, and types of servers could vary widely. Also, I haven't included the cost of setting up the T1 or whatever Internet connection is chosen. If remote access is vital or if 100% Internet uptime is vital, you will want a backup Internet connection of some sort, even DSL.