I'd agree with Boris but I'd tend to favour the Poweredge machines as they are designed for continuous operation. And watch for the Dell deals, you can routinely find a competent box for budget prices. Sub $500AU in my case. I normally buy the budget Dells as offered and throw two large capacity SATA disks and an intel NIC which wont add much at all to the all up purchase price.
The other advantage is you can order Poweredge online without an OS, With Vostro or Optiplex you will have to specifically request not paying the Microsoft tax when you order.
Small form factor is a nice aim as is fanless, but you'll probably be paying a lot more for a lesser machine.
And Don't tightly build in any device that doesn't not follow a standardised form factor. Ultimately you'll upgrade any computer and you'll be upset if replacements do not fit.
Your using a separate room so noise should not be a consideration. But understand that in a domestic or home office that the fan noise from from some Servers, Switches and UPSs can sound like a launching Lear Jet on calm summer evenings. Dell T105 is a real winner for a whisper quiet box. I'd also forgo anything not absolutely necessary. I usually load SME from a loose CD drive as you'll only need that drive fro the few minutes you spend on your your initial load of SME.
Another consideration is to use a NAS based backup or storage device in a seperate location in the dwelling. It offers some protection in terms of theft or accidental fire or water damage.
While as much ram as you can throw at it is always an agvantage, I'd think given your suggested application that 1Gb would a comfortable minimum. I have lightly loaded SME's quite comfortable on 256K but 1Gb or better would be more than comfortable.
I wouldn't be too concerned about power consumption, from basic servers, A basic but competent Server and a switch are unlikely to even be noticeable on a domestic electricity bill. You don't need to be running monitors a etc, just the basics.