Sounds good Chris, you have a nice bit of cash there. I think 2nd hand certainly is the best way to go if you're happy to wait.
The ED100 would be a great scope on a HEQ5, and these mounts come up second hand quite often (not many ED100's around though). From what I've seen around the place lots of people that buy anything less than a HEQ5 often end up upgrading quite soon, whereas many people stay satisfied with the HEQ5 for many years (me being one of them). If you really want to do astrophotography at or near 1000mm focal length I think the HEQ5 is the minimum mount you should consider.
I seem to remember there being a great thread on this site regarding a comparison between the ED80 and ED100 (might have to do a search)... I seem to remember the general consensus was that the ED80 was almost as good for imaging (better for some things) but substantially cheaper than the ED100. Also there is very good info and support regarding the ED80 (reducers, flatteners, focusers, how to get the most out of it etc). The ED80 seem to come up 2nd hand far more often too.... Any reason you prefer the 100 over the 80?
Just my 2 cents
