ALPPPIIINNNEEEEEEEE!
I look for anything that puts out and has high voltage Preamp outputs (4v or more!). Make sure you have Preamp Outputs for each amp you will run (i.e. If you will run a front speakers off an amp make sure you have front preamp outputs). If you aren't going to run your speakers off an amp (aftermarket or stock) i would suggest a head unit with at least 20w RMS x 4 channels. I had the Alpine CDA-9855 and it was a great head unit for the cost.
This guy might sell his for 200 or so...make an offer if you want.
http://minneapolis.craigslist.org/ram/e ... 53604.html