Hi David,
You have more experience with ebay than me so I won't comment there.
In terms of mirrors, I would always go for the pyrex mirror. The manufacturer is more likely to have been able to make a quality product. Same thing with F ratio. F4 is much harder to make well than F6. In an 8 inch mirror I don't see many advantages with an F4. The F6 will have a similar wide field if you use a long focus eyepiece. If you build an F4 you will need a coma corrector and need to build a bigger mount to bring the eyepiece to a convenient height. I can't comment on the quality of these mirrors as I rarely buy mirrors. I still make my own. The ebay mirror is for a quick father son project. I didn't want him to lose interest waiting for the mirror to be finished.
When thinking about your design, make sure the top end is as light as possible. The Guang Sheng focuser may be a problem there. I would recommend the Kineopics HC 2 shown here:
http://www.kineoptics.com/HC-2.html
It's weight is negligible and it is a very good focuser. I use one on my Travelscope. Similarly use something like a quickfinder as a finderscope. It's also small and light.
I was thinking that a single pole design might be a good way to meet your requirements. Ross Sackett has some examples here:
http://www.stardazed.com/MyTelescopes.html
Chris (scopemankit) built a very nice single pole scope. There are photos of it on this forum somewhere.
Hope that helps,
Rod.