Also, keep in mind that you may have an opportunity to buy a van REALLY cheap with a top on it. I have seen a bunch of sub-$5k vans that have lots of rust or don't run. It's worth the price to get a top.
That being said, have you contact SMB texas? They will do a top for you for about $8k. Used prices are usually around $3k, so that $5k difference may be a deal killer, but it also may be the difference between waiting 2-4 years to find one or getting one now. They can usually schedule a top-only install within a couple weeks.