As for the software costs, if you're not using MAPS then you need to get signed up. Subscribe to MAPS and you'll have more options than you need for server options. We're currently running a Server 2008 machine (non VM), and sometimes really wish it was for portability/upgrade/backup and migration options.
We have our current nox running mirrored 160GB drives for OS, and mirrored 750GB drives for office in house storage, documents, accounting stuff, etc. We use the server 2008 machine for DC, Exchange and a TS Remote Gateway. Would probably run Server Essentials 2008 if we had the option.
On the repair network we run a Synology NAS with 4TB storage (which is also accessible from the private office network). We store images and client backups on it, and it's fast, simple, and integrates with the domain for security control. We're working on a system right now which will put the database for our repair tracking software on the Synology as well.
If you figure ~$500 for MAPS, you should be able to put together hardware for a box for right around $500 to accomplish what you want. I'll warn you, we have 6GB RAM on the Server 2008 box with Exchange and we're getting ready to migrate it over to a VM box, but this time we're giving it 8GB because it's using the 6GB every day. If you go with SBS or Essentials make sure to get at least 8GB RAM. Bottom line, Server 2008, DC, Exchange = memory hog.