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Adrian. the 10" wont go onto a HEQ5 nicely... it will probably be fine for visual, but wouldnt be stable enough for photography.
As for the comments on long vs short focal length....
A shorter focal length will give you a wider field of view, an longer will give you narrow field of view.
Both are fine for astrophotography, but this all depends on your mounts alignment, tracking and stability.. A long focal length will require more stablility, because it takes less movement at long focal lengths to cause star distortions and blur. tracking/alignment errors at shorter lengths will be more forgiving than with long.. for example... a few weeks ago, I took a few photos of M20/M8 with a camera/lens, however I had perhaps the roughest polar allignment ever... I basically set my latitude, and pointed the mount in the general area of the scp, then started taking photos... I took one through my 6" refractor (1200mm focal lenght) and got nothing but star trails through a 1minute exposure. I then piggybacked the camera and a 200mm lens (very short focal lenght for astrophotography) With the shorter focal lenght but the same shocking alignment i was able to take 1 minute exposures with very minimal star trails...
For your first shots I think something around 500 ~ 700mm focal length will be the best trade off between good image scale of nebs/galaxies vs hair pulling tracking problems.
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