I use my BenQ for 3D only but the contrast/black level leave much to be desired. Would the Optoma UHD51 contrast be much of an upgrade over my BenQ? I'm not looking for JVC quality contrast. I prefer DLP for 3D. I would like to spend no more than around $1,700 but would prefer less.
It would be going sideways to slightly backwards, both have near the same native contrast, but the Benq has a DI.
There isn't a noticeable contrast improvement in DLP tech for this price range, you would have to look in the used (non-4k) market to find some decent DLP contrast.
Runco or Sharp xvz-30000, but those are 1080p projectors and only the Sharp has 3D of the two.
I would suggest a used JVC RS-46 or Epson refurb 5040ub + cheap DLP for 3D, that would keep you in your budget.
Get the Epson first, then find a refurb DLP later to add and do a dual-install.
You could also look at the Benq ht3550, but I would not expect much of an improvement, more of a sideways move, but Benq's DI algorithms may work better in their newer projectors (not sure).
You can get a used JVC RS-46 for incredible prices if you do not need a lot of brightness.
Use that for 2D (huge improvement compared to what your used to), and then get a secondary DLP for 3D.
You should be able to find a used RS-46 for $1200 or less (maybe even $800ish?)...
Or even better, the RS-49/4910 is the best bang for buck in the used market, as it supports 4k e-shift at least.