I got some very good deals on some Dell SC420 servers. $225 per box. Can't beat it for a P4 3.0. I bought 2GBs of ram from Newegg for about $150 and now have a kick butt server.
For backup and storage, you should look into the Accordance ARAID 2000. I have two in my new network servers (one in the file and print and one in the SQL server box. Makes backup a no brainer, and I can keep multiple drives running in RAID 1 and can pop one out at night and take it off site with me. Plus, for the price, this arrangement can't be beat! They also make an external USB version if you prefer it over the internal version.
Also, look into Small Business Server 2003 Standard. You don't need SQL so you don't need the Pro version. The features in SBS are awesome, including a Wizard setup to run Exchange Server internally. Or you can always get Excahnge at 1and1.com for $7/month. That's what I do. 
I defer to the other experts here on the other specifics as I had a friend setup out network. I'm just an expert at buying paying for the stuff. 
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-'13 E93 M3 Cab, Interlagos Blue, Carbon Black leather, DCT, all options, and fun, endless fun!
-'13 X5 35i M-Sport, Carbon Black & Cinnamon, Dinan Stage 2!
-'15 Toyota Sienna Limited Premium AWD, Silver & Grey, Nav & BluRay, and less lame than the 2011 it replaced
-'07 Mini Cab S, Laser Blue, 6sp, endless fun that ended (sold)
-'04 X5 4.4i, loaded, loved, modded and sold
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