A SSD will be a massive improvement in your performance. If the $50 price difference is important, then used is OK.
Most SSD's will be long obsolete before running out of nand update capability. At least when used in a desktop environment.
So used is probably ok. S.M.A.R.T. data can tell you how much of the update capacity remains. Intel SSD toolbox can tell you that, I don't know about how to read the data from different SSDS's.
If you buy from Amazon on an online retailer, you are buying new, and the price per gb will be similar everywhere.
Prices are coming down, and are close to $1 per gb.
Be on the lookout for some rebates on Intel 320 drives. I think they are trying to clear out older inventory.
For most users, performance for the os will be just as good as the newer 6gb capable drives.
If you buy from E-bay, look for a used drive from a seller with good feedback.
Just verify the sellers return policy in case the drive is defective, and pay with paypal.
Any Intel SSD will be good, even the older X25-M models.
Samsung 830 is also good.
I would avoid older generations of any other vendors, some of which had firmware issues.