You have a collection of criteria which I think are putting you out of the price range you're shooting at. The major one, I think, is the insistence on 4 valves.
But you know what? With a decent BBb tuba, you don't NEED 4 valves. So think about dropping that requirement. If you do, it will open a lot of opportunities you don't otherwise have.
Consider something like this: http://www.1800usaband.com/products/view/5571. There are also a couple of interesting 3-valve BBb horns for sale on the Charlotte and Winston-Salem Craigslists -- including a nice-looking Conn 15J with silver finish. Both are under $2,000 and you might get them for less if you make an offer.
Otherwise, keep spending your money on the rental until you get lucky (if you think that's cost-effective!), and keep looking. A Cerveny BBb 4-valve rotary sold recently on Tubenet for $1,800. But deals like that don't come up very frequently.
EVERYBODY wants a decent playable 4-valve BBb tuba for under $2,000. Good luck.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)