Firedog's offer isn't a bad deal. You should be able to find a jacket style BCD used in GREAT shape for about $250 or less, add $50-75 if the inflator is also your alt air source. If you're looking to go with a bp/w set up, you're looking at about $350 used and you'll need an octo on your regs.
I've found some great deals on craigslist, ebay and on here. Just remember that if you buy used regs, you'll need to spend up to another $75+ to have them serviced....even if the seller says they were just serviced (unless you trust him/her with your life). The other catch to buying used is you've never tried it out. You might buy a BCD for class and hate it. Too late...you own it. Hence, the advantage to renting for OW class. Good luck.