As Torqueboards said 2 x 3S wired up in series will probably be easier since your board is fairly low profile (due to small wheels and lack of risers.
I've been with my Hobby King 150A ESCs which are much cheaper than the EZ Run (here at least) though I'd probably go the EZ Run if the prices were very close.
If you want to ensure range and don't have much in the way of hills to deal with I'd stick with 15T/36T setups (35T is a non standard size). I prefer not to go below 15T because it allows you to go as short as possible on the belt while retaining sufficient belt wrap. 14T and less you are below spec. Shorter belts mean a lot less slip at lower pretension making it kinder on bearings, belts and the feel is just a little crisper.
The 36T will mean you top out at 31km/hr (19.3MPH) with 260kv motor and 6S. Going more than 6S will make add a bit to your budget as ESCs are pricier above 6S and of course the extra battery cost. If you got to 30T (what I run but with 190 or 210kv motors) this will give you a top speed about 37km/h (23MPH) BUT I would stick with the slower 36T option. It will give you more climbing ability, make take off a little smoother and I think you will find closer to 20MPH more pleasurable on such small wheels.
Yes, the 8-10S motors will be fine on 6S.
Make sure your batteries are 30C minimum especially from Hobby King - partly because you need the high current uphill and partly because cheaper batteries tend to overstate their C rating.