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Hi Hoopa! Welcome to IIS!
I don't know of any telescope dealers in Canberra - I doubt there's any specialised ones (or we'll have heard of them). The best bet might be a camera dealer or something? But don't be put off shopping from Bintel, Andrews, MyAstroShop etc. Plenty of people order over the phone through these guys and have not been disappointed.
Where did you hear f/4 was better than f/5? No one focal ratio is "better" than another. There are pros and cons to each.
f/4 will give you a wider field of view (shorter focal length), but will be much less forgiving on collimation and eyepiece design. Collimation will be CRITICAL in a fast scope and your average widefield eyepieces will look horrible in half of the FOV. The tube will be shorter (shorter focal length) meaning (perhaps) a more comfortable eyepiece height.
f/6 is easier on collimation, easier on eyepiece design but the tube will be longer and for a larger mirror you may need a small step ladder (unless it's an 8" f/6 like the GSO's).
f/5 is somewhere in-between. Most of the commercial scopes of 10 and 12" aperture are f/5. Collimation is critical as with every newtonian, and widefield eyepieces are usable but the view around the edges is still not great unless it's an expensive model like the Pentax or Nagler.
Planets or DSO's make no difference. It doesn't matter what the focal length is for planets, you'll still put in a high power eyepiece to get the magnification you want.
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