1. The native ratio is no relevant, IMHO, because the movies themselves are in different ratios, so the ratio of the screen only gives you an exact fit to one type of movies.
Because you need to mask the top and bottom of the screen anyway, it doesn' t matter what ratio of screen it is.
Look for the article about masking.
2. As Ofer said, it depends on the projector.
Try to aim that the width of the screen won't be bigger than 1.5-2 times the distance of you from the screen.
3. Depends on the budget. I can name few screens for a few thousands of Dollars if you wish....
Check the special deals.
4. Depends wich brands, but as mentioned here, Softec are a good place to start.
5. Not really.
If you bare in mind that the projector aims it's picture a bit above the lens line, this mean that if you put it on a high shelf, you will need to put it up side down anyhow...