Have you considered the HTPC route? $1500 can get you a scaler, DVD output and gaming machine...
If you go with just a doubler, is that enough for the Pro900X? (I'm getting a NEC XG85 and it has to be driven with more than just 480p, most people recommend 960p or 1080i for it...)
On the dedicated scaler front, I've found many used Quadscan scalers in the ~$1000 range, and many DVDO iScans in the $400-$700 range...
HTPC=Home Theater PC. A computer. You can set the output to 72hz and a nice high resolution that your projector supports and you won't even have to deal with 3:2 pull-down!
There is also dScaler which is a software based de-interlacing program for non DVD or HD sources (both of which can be done directly on the PC.)
If a PC is more trouble than you want in your home theater, then you'll want an external scaler/de-interlacer.
The quadscan would be a good scaler, but I think it only scales, not de-interalces... double check that to make sure though!
An Iscan is only a de-interlacer, and not a scaler. I don't think you'd be happy with the resolution it outputs.
Pop in over at the AVS forum, there are a ton of people with FP setups there with a myriad of different scalers etc.