The general answer is how large of places do you play and how loud and how deep do you want the bass to be? Those subs are punchy, but do not go low. They might do fine-depending on those particulars.
Be wary of buying used speakers-if they have any problems and you have to start to get them reconed/replaced/diaphrams etc., you might have been better off buying new speakers.
When listening to used speakers-do a couple of tests. Listen to them (espically the bass/mid) at very low levels-just above what you can hear and listen for buzzing sounds-If you hear any-be wary. Listen to them at high levels and listen for any other "artifacts" that might be coming from the speakers. If you have access to a sweepable signal generator, then use it to swep the entire operationg range of the speaker and listen for any wierd noises/rattling and so on.
If you hear anything strange during these test, the speakers might be on their way out-or it might just be a cabinet problem, and be prepaired to spend some extra money fixing them. If you don't hear anything, it does not mean that the speakers are totally fine either-they just haven't gotten quite to the damaging point yet.
Often times you can get a good deal on used speakers, but since they are a mechanical device, they are more subject to mechanical abuse than say an amplifier or mixer would be.