You got a good woman, she'll save you money in the long run.

The D201GLY boards seem to be the best best. There's a pretty long thread detailing the construction of a headless rig using that board. I think it was by superczar.

Should run on 15 watts without hard drives, add about 10 watts for each drive.

A Sempron LE-1150 and 690G combo will run up to 35 watts or so, and will be more expensive to start with too. I'm assuming you don't need the extra power of that combo.

Edit: the board (with CPU soldered on) is about 3K, IIRC. You can be well within your 10K budget with the machine.