Given the scope of the project you're proposing, I would recommend you invest in cylindrical optics for your lens. They are significantly more expensive, but especially on large screens they don't give up as much to chromatic aberration. Maybe something like this
could be worked into the budget - I don't think you'll need $10,000 for the projector alone.
Along the same logical lines, you are in a position to really benefit from using multiple side surround loudspeakers in an array, like in the commercial cinema. This will get each row of seating a better surround presentation, but means more speakers, obviously - as well as more amplification - but also special processing and probably pro calibration. Most people find that for two rows of seats you can make a reasonable compromise about surround location and calibration, but I'd argue that three rows present a real challenge to good setup without multiple surrounds.
You can definitely keep using the amp and maybe the projector too. It'd be nice to have more light output, but with light control you might get by with a new bulb and a screen with some gain. You will definitely want more sub(s).
If you're going DIY, jautor is right - start with defining the seating area. Build the speakers around that and see what screen and projector fit from there. If you're not going DIY, now is the time to consult at expert for the layout - it could be the best money you spend on the whole project.
What about major mechanical - HVAC and sound isolation?