Well, when you are considering the difference between a scaler and a line doubler (really it's a deinterlacer these days since the unit does not try to interpolate the missing lines of information in an interlaced field, but combine two fields into a progressive frame, but I digress) is the quality of the deinterlacing and the line density on the screen for a given resolution.
For instance, on a 100" 4:3 screen, a 480p image will only yield 8 lines per inch, whereas a 600p image will yield 10 lines per inch, resulting in a smoother picture with fewer visible scan lines. It has been my experience that anything greater than 10 lines per inch of vertical height will look fine from greater than 1.5 x screen width. With a 480p signal on a 43" high screen, you will get 11.1 lines per inch. From your seating distance this should appear very smooth and film like. Your projector (and mine, I have a 1271) has a sweet spot of around 600p. You can drive it higher, but the image will begin to look soft. At 600p, your lines per inch are 13.95, which is more than enough, but again, we are now speaking about a matter of degree. 11.1 should be more than adequate.
And then there are the other, more standard considerations to take into account when selecting a video processor: number of inputs if you need it to do switching, vga passthrough for a possible computer or HD tuner connection, picture controls, aspect ratio control, ease of setup and use, remote control, price, and so on. If something like the Iscan Pro, meets all of your needs in the above categories then it will likely give you the picture that you're after, and it should be sharp enough as well. Of course, the only way you'll ever truly find out is to try one out in your system. You may also want to keep an out for the FX2 from Immersive/Vigatec and the Videon Omega One. Both are deinterlacers with the Sage/Faroudja chip in them, which should give them supior quality to the Iscan.
Edited for speelllling mistakes.