I also agree with buying a package of machine and frame. Look for something sturdy. If there is a join in the long roller bars, grab the bar there and give a wiggle. does it move? The other thing to look at is the carriage. Are the wheels metal? what to they roll on? if it's plastic it will need constant replacement. If you are looking at a stitch regulator ask to see the sensors. Do they look kind of flimsy? If you are pressed for room, you may also want to see if the frame can be fitted with wheels.