As for hardware, if you go with SBS, for 4-5 users, Any basic server should be fine. ALL SERVERS (with rare exception) should include RAID 1 or RAID 5 and all Windows Servers should be purchased with 1 GB of RAM at least. Beyond that, I wouldn't worry too much about the specs. I will throw out there that with today's available technology, the server should have the ability to use multi-core CPUs (you can start with single core, but be sure it supports multi-core later) or supports multiple processors (again, you can start with one, but having the room to grow to a second one if performance grows is very important in my opinion).
And any SERVER hardware you buy should be purchased with a 3 year AT LEAST Next Business Day parts warranty - that's if you KNOW how to build computers. If you don't, you should be getting a 3 year On-Site service contract. And depending on how important the server's work is to your business, you might want to get a 3 year 4 hour 5x10 or 7x24 response contract. The better the warranty and response, the more the money, but do you want to be sitting there scratching your butt in 18 months when the motherboard dies and your out of warranty? Or while you wait until tomorrow for someone to show up with a replacement motherboard because you didn't get a 7x24 4 hour response contract on an IMPORTANT server?