Hi John
HEQ5 is a fine mount - I successfully used if for photography (up to 1200mm) for over two years.
Honestly, it would be much cheaper to get a guide camera and a guide scope than buying a high end mount with encoders for unguided photography. I do not think you could do unguided photography with either AZEQ5 or AZEQ6 at 600mm, you would still be throwing away lots of subs.
There is currently SBIG SG4 for sale on this site - it is a self contained guider so you would not need a computer in the field.
Just my two cents.
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