This question seems to come up on a regular basis, there is some good information in this post -
http://www.quiltingboard.com/main-f1...m-t221787.html
Also, do a forum search for "longarm search" and you will find a bunch of threads on the topic.
My main advice is to try as many as you can before investing. Everyone is different, and will need different things. When you see many machines you can identify the features you want and hopefully get the one that fits your needs best. For me the top two items were hydraulic legs and a stitch regulator.
For used machines, the best websites out there are House of Hanson and Longarm University -
http://www.houseofhanson.com/formerlyowned.html
http://www.longarmuniversity.com/machines_for_sale.htm
Good luck!