I was thinking of two per side. The cheapest place I found locally for single hung windows was Mr. 2nds. Their new units and work fine.
I understand building on the cheap. When I built mine I used all the leftover windows we had laying around (screw ups from job sites) but I still had to buy two windows for the one wall. The door was a special Home Depot had for $99. The shingles were leftover from jobs as was the siding, soffit and metal.
I found some windows (they are replacement windows) listed on Craigslist:
Used double hung with screens
4 are 31 1/2" x 52 1/2"
3 are 30 1/2" x 53 1/2"
Do you think they would be too narrow? They are priced right (about $20 a piece) for me, unless you think they wouldn't work out very well?