A handy (free) tool for checking out how big your images will be is CCDCalc. You can setup your camera and telescope options and pick astronomical objects and it will show you how big they'll be in the frame.
You're opening up a can of worms here Am.
Coma is an error that is inherent in Newtonians and is determined by the focal ratio. Stars in the middle are round and stars at the outside look like comets. If you have an f/4 telescope, the diameter of the circle on the image plane with round stars is about 8mm. For an f/5 scope, that's 12.5mm, f/8 is 32mm, etc. The sensor of a crop camera is 15x22mm so coma is a non-issue for an f/8 telescope but is substantial for a f/4 scope.
A "coma corrector" or the much more expensive "paracorr" aim to just resolve the coma with no change in focal length. Another option is to use a much cheaper Barlow (like a 2x teleconverter) which multiplies the focal length by 2 and therefore the focal ratio for your f/4 scope is now f/8. No more coma but only a quarter of the field of view and longer exposures.

The other optical doo-dad in our bag tricks is the "focal reducer". As the name implies, they are the opposite of a barlow lens and reduces the focal length. They are most commonly used with Schmitt-Cassegrain Telescopes (SCT). The SCT has the advantage of aperture and focal length in a physically smaller package than a Newtonian. However, they are usually quite slow at f/10. eg. A 8" SCT has a focal length of 2032mm. If you put a 0.63x focal reducer in the image train, that becomes a 1280mm f/6.3 telescope. SCT's don't have issues with coma but do have a curved field so the focal reducer is usually designed as a reducer/flattener to fix that at the same time. There is also a 0.33x reducer for SCT's and it makes it a fast f/3.3 but it's not suitable for DSLRs.
There are also reducer/flatteners for refractors and they're usually matched to a particular focal length and ratio.

It looks to me like the f/5 scope is coming out ahead of the rest at this point and while a coma corrector will be in your future, I don't think it will be a limiting factor for a while.

Steve.