Today I have had to educate myself about backfocus as i want to enjoy the frustrations of Astrophotography and have built my kit up to the point where i am almost ready to take my first photos. So back focus is the distance the sensor in the camera has to be away from the rear of the flattener in order to achieve pin-point focus / flattening. I can only talk about my setup as being new its the only one i have any experience with, but i am sure the lessons translate to other brands and setups.
Before you buy, decide what you want to do.
Do you; want to look at stuff or take photos of stuff? Or do you want to do both, but not as well as you could do if you dedicated yourself to one field. I decided i wanted to be able to do both reasonably ok and accepted that with my first scope (WO 91 FLT) that I wouldn't be able to view the planets as close up as perhaps I would have like to, but that i would be able to take clear images and move on to astrophotography (as is my goal).