Our original plan was to frame out a fixed screen - yet while in a discussion with a prospective contractor - he mentioned instead going with HD painted - framed out as well to resemble a screen.
What is "HD paint"? There are a number of 'screen paints' available but I'm not sure what they are actually referencing.
As Ryan and Tony noted, with 'screen paints' the wall preparation is often critical to the resulting image quality, any variations, seams, etc. in the wall can translate to the projected image. And with some of the 'screen paints' you have very limited knowledge of what you are getting as they may not provide gain, vieiwng angle or other technical data.
Not knowing the specific product and many 'screen paint' products offering limited or very questionable technical information makes it difficult to answer the kind of questions you seem to have. And unless the technical data is available and can be accepted as being valid, then there may be no way to answer them other than to try it. That being said, there are many churches that have been quite happy with 'screen paint' screens or even regular painted walls, it's all a matter of expectations.
You might want to verify what surface prep in included in the screen paint cost offered or whether that might be addressed as a separate effort and associated cost to be provided by others.