You might want to check with one of the bus conversion groups out there such as skoolie.net. I've seen similar questions posted there before. Frankly, assuming you don't require a brand spanking new bus, with that kind of budget you should be able to get exactly what your looking for with a little patience.
When I was shopping for my bus to convert, I saw several that matched your described needs for under $10,000 on auction sites like ebay, publicsurplus.com, and govdeals.com.
It sounds like what you want is a small shuttle bus, probably on a ford E350 or E450 chassis. These are the little busses you see at the airport for hotel shuttles. Something like eBay item # 320340473790 might do the trick for you.
I hope this helps,
jim