I use a 7mm in my 10" GSO for high-ish power. Most nights, this either works or is very close to being acceptable. I also have a 3.5mm, which on most nights does
not work. So an EP that gives you about 150-180x isn't a bad choice. I have heard that the eye relief on the 82° ES series can be a bit tight, at least when you want to take in the whole AFOV. Should be OK though if no glasses are involved. Perhaps someone who owns one can say whether this applies to the 6.7 and how "bad" it really is. Also seeing the whole AFOV becomes less important as the exit pupil shrinks, since the darker image prevents the "immersion experience" from happening, IMHO. But a large AFOV is handy at high mag because you have to nudge the tube less often.
Eyepiece cleaning - there are some good threads on this forum on the subject. First time I've heard that they need to be cleaned professionally
. You will find that simple and readily-available cleaning products are just fine. Think camera lens cleaning. I use isopropyl alcohol on mine, no probs so far.
Observing site. The inner parts of town are pretty evenly polluted, so to speak. I would think it makes little difference travelling 30mins in any direction. In saying that, Sydney is actually pretty good compared to some other cities I've observed from. Alex might know more on this. The man knows his stuff. When observing from Dunedin, I find the "Town Belt" (a park) to be acceptable if I don't want to travel. So a park or some other green patch near where you are might give a slight improvement. It helps with the seeing too.
The MacArthur Astro society has an observing site available for members and their guests at The Oaks, about 1hr from where you are. No sure about their policies & conditions of use. Check
http://www.macastro.org.au/mas/
Cheers
Edit: The ES that interest me most are the 68°. The one that impressed me most was the 30mm 82° though. 100° Series is great, but probably need a coma corrector in an f/5 scope. The shorter 82° I have not looked through yet.
Had a look around. Adrian (AG Hybrid) might be the man to talk to re the 6.7 ES