Pick up a used furling genoa for dummies ?

[Joka]

Hi all!

My new yacht needs a new genoa. I've been looking around and there seems to be some fair used sails around!
On the paper, it looks ideal. But is it really? I dramatically lack of experience, so any help welcome

My questions :

- apart from a reduced lifespan, is there any other drawbacks?
- how bad can a used dacron sails be difformed? Is it a big concern? Is it any better if the sail has a luff foam pad?
- is there any hidden defects?
- Dimensions : how much will a different sail affect performance, and with how much margin? (I.e a sail with a luff 5 or 10% smaller than the maximum, or a leech that is too long or not enough with a high or low tack)

Thanks in advance for your advices!