You may find the cheap ones are from the outgoing wireless standard - which means that in many regions they will no longer be legal, and in most regions will have a great deal more interference as the frequency range they use is open to more devices. Can be worth picking up one of these, as they are cheap, however you may not be able to use them in some venues.
Much better to go for a new system. And depending on what sort of music you play, a bi-directional one can be very useful (for in-ear monitors). If you sing at all, have 2 channels sending.
Try and avoid compression - the better ones tend to state 'wire quality' as they keep the sound as close to wired as possible.
Diversity is also a very useful feature - in a closed room, you will get interference from your own transmissions, and in some locations this interference may lead to no signal. Diversity is the use of two antennae so that if one is in an area of destructive interference, the other should still have a signal.