How tall are the ceilings? Also how important is 3d to you?
I am currently looking in the same price range as you and would strongly recommend you check out the jvc x35, the panasonic 8000 really pales in comparison to my eyes. I'm not a fan of epson as I have not had pleasurable experiences with there build quality, so I won't comment on there projectors beyond that. I can't comment on Sony projectors as I don't have enough experience with them.
If you mainly watch movies the general recommendation is to go 2.35:1, if you watch mainly tv go 1.78:1. Personally I prefer to go 1.78:1 as I find I tend to like the extra height in those films and not miss the "extra" height in scope films. I think most would disagree with me though. Another thing to consider with goin 2.35:1 or 1.78:1 is that there are films with aspect ratios that change between the two (Tron Legacy, The Dark Knight, The Dark Knight Rises, and i'm sure there are some more). With a 2.35:1 setup you will also have to buy a scaler for such films which is rather expensive.
As for screen size it is hard to say. I personally prefer a little larger then the THX guidelines, some prefer a little less and some like what THX recommends. I would focus on your projector and then get a large white cloth and play around with screen size at first so you can find out what you like. So what you consider to be an 'immersive' experience will require some trial and error on your behalf. The below is a great website for some guidelines, don't however treat these as rules, rather use them to get an idea for your screen size.
There is also a thread on it on the below link. I would recommend you play around with some whit clothe and your projector sitting the same distance as you will be from the image to find what size suites you.
As for the screen relative to the seating, just make sure neither row requires you to tilt your head more then 15 degrees to see the top of the screen and you shall be fine. As for what screen to get, that is debatable, just remember paying more doesn't necessarily mean better. I have seen $10000 screens that provided a worse image then a diy screen using . Screen gain will depend on what projector you get, how big the screen is and your seating positions (will any seats be to the side of the screen or all within the edges of the screen). In other words the screen excluding perhaps whether you go 2.35:1 or 1.78:1 is more or less the last point of consideration, everything else should come first.
Most importantly, invest in some sound proofing before you plaster as this will make a massive difference to what you can do audio wise in your home theater. You of course want to run all your cables prior to this as well.
I am currently drawing a basic plan for my home theater which I can share with you if you are at all interested, this is another thing I would recommend you do for your own home theater as well if you are designing and building it yourself.
[edit: added more detail, I don't believe I have made any mistakes but remember to do your research and once you think you are done, do some more. The results will be worth it.].