THe Schmidt Newt design has been commercially done so the design basically works.
I'd consider a 26" or so primary as this will give a good 2" FOV without any problems.
There is really no way to easily correct the off-axis aberrations but astigmatism won't be much with the design, not to the point where you will be seeing it over the astigmatism of the eyepieces that you'll be using. For film camera work, astigmatism is the most benign of the errors as the star's location is still the same as it really is but it is smeared a bit more in one direction than the other which can be calculated for in any star positioning calculations.
BTW, BK-7 is a common glass in the optical field and is about the cheapest of the glasses available. If you want to go cheaper, you go to glasses that aren't rated for optical performance.