I don't really have a specific brand recommendation but here are some issue that I've encountered or am aware of:
An OAG will?add to?the back-focus distance and?the effect of that?depends on the scope (Edge-HD or Classical?), external optics (reducer?), filter wheel, etc.??In particular, a?reduced Edge-HD is very particular about that distance and it may be difficult/detrimental to insert another gadget into the imaging train.
Also, the mechanics of an OAG can range from simple to sophisticated, particularly the ability and ease of camera rotation and pick-off mirror distance and rotation. Also the focusing mechanism for the guide camera can range to virtually non-existent (simply slide the cam in/out) to fine-control.? And some OAG have integrated or compatible filter-wheels, etc. There are motorized OAG.? Obviously these issue dictate the cost.
Top-end OAG are heavy.?The C-14 is heavy enough that it probably won't be an issue. But it does potentially stress the mechanical imaging chain and?might induce flexure, which is not good.
It can be a real pain to locate guide stars and if you rotate the entire unit (incl camera) then you have to take new flats.? So it is important to pre-plan all imaging fields prior to imaging.?Use "The Sky" to?map your shots.
There are some reasons that DSLR does not?benefit much (or at all) from long sub-exps (e.g. even in a dark sky the sub-exp can become dark-current limited, similar to sky limited; also the uncontrolled temp can be problematic for long sub-exps).?If your mount alone (or with a modest external guider) can handle a?minute or so?then an OAG might not?make much of an improvement.
Stan
StarManMike
George Dunham