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Good advice from Dunk and Alex re the kits, these only contain a range of plossls and some coloured filters. With Plossls usually ony the 25 and 15 are really easy to use and the filters are only really useful if you are a dedicated planetary observer. For DSO's, you want OIII or UHC type filters (if you wasnt filters at all!). In eyepieces a couple of good quality EPs will cover most needs.
Coma is, as Alex said, not really a big issue for visual use. I have an 8" f4 and the coma is noticeable but not in the centre of the field where you are looking. On the 8" f5, 12" f5 I used to use and the 20" f5 I never even noticed it. Save your money and only worry about correctors if you really feel it is an issue once you have a scope.
As far as a choice between f4 and f5 some factors that should be considered are overall cost, size and portability.
An f4 gives wider views and is a shorter scope but will be more expensive (all things being equal). As I mentioned I have an 8" f4 dob but it specilfically designed as a very portable unit so the shorter focal length was a must.
An f5 will give narrower fields, longer tube or struts and lower cost.
Hope this all helps
Malcolm
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