Hi John,
My limited understanding tells me that you should be more concerned with imaging resolution in arc seconds per pixel rather than just with the focal length by itself. With HEQ5 I would stay above 2 arcseconds per pixel for DSO imaging (although it is possible to successfully image with this mount at higher resolutions). I would aim for 2.5-3 arcseconds per pixel to start with, as it puts less demands on your mount and thus makes imaging more enjoyable. You can always apply drizzle process to your data to increase resolution of the final image.
As for the F-ratio, the lower the number the faster the system and thus the shorter the subs, but there are implications in terms of increasing demands on proper optical alignment and colour correction (refractors) for faster F-ratios, not to mention taking into account vignetting and wavelength shift for filters making narrowband filters less effective at extremely low F-ratios.
Exposure times depend on many factors, but some software estimates optimal exposure times for given target taking into account your camera's characteristics, resolution etc. for example SGP gives such estimate after capturing every frame, allowing for adjusting and experimenting with exposure times. So perhaps your 110mm refractor with a reducer and QSI683 would make a nice imaging apparatus
Planetary imaging is out my my "comfort" zone..., so I won't comment on that.