How about a Singer Featherweight 221. Get one of the older ones, not the new plastic ones. They are small, very portable, very reliable. They do a straight stitch, that's it, but is great for piecing. I've gotten all mine from eBay for less than $225 (which included shipping & handling), or you can get one from a reliable dealer for $350-$400.
If you want a newer machine that has a few bells and whistles, the Janome Jem is a good one, as are the Berninas. My suggestion is to start looking around and trying out the machines.