Here's a good place to start:
http://forum.rotojunkiefix.com/showthread.php/5803-RHD-s-Latest-Top-150-Roto-Prospects-(End-of-Season-Edition)
Are you drafting 18 rounds of rookies? Or 18 rounds total? That's a big, big difference.
I would try to avoid guys who are still in rookie ball. Even if their ceiling is pretty good, a lot of guys can crater unexpectedly at Double-A.