Hi Ben and welcome!
Re "I'm not really wanting to have to buy another scope down the track if I can help it"
I suggest that you console yourself to the fact that you will buy and sell more scopes in the future as your interests and desires change. It seems to me that 99% of amateurs on this site do that. Don't panic - in Australia, resale value is fairly good, so it is not money wasted. In NZ??
For planets, a quality, smaller aperture refractor, would do fine - but the $s will be more than you seem to have budgeted, if you are looking at 6" and 8" reflectors on dobsonian mounts. For observing planets, f6 or f8 is much of a muchness I expect and the somewhat better resolving power of the 8" will be worthwhile. But is it worth twice the price - I don't think so.
The moment you mention observing DSOs, aperture becomes a prime consideration. In that case, you should definitely hold out for an 8" or even 10" reflector that you can afford.
Keep asking questions. Have you found anyone local with scopes that you can look through so that you can decide?
BTW, the changing focal ratios are probably just a way of keeping the size of the scopes manageable - not too long, nor too short for a dobsonian mount. It's not something intrinisic to the mirror diameter.
Eric
