Re the balance issue : with a standard Obsession-style dob, you can fine tune the balance near the end of construction by shifting the mirror cell up and down. This is not possible in some other designs.
If you make a scope like Babcock's 12 incher, then you can build the top ring and mirror box etc.first. But, you need to work out the weight/distance maths before you can decide on the size of the side bearings, which lead in turn to the dimensions of the rocker and ground board.
As I got all my parts pre-cut, I had to work out the weights right from the beginning.
If you are thinking about DSC then having central pivot points is handy. Horseshoe or crescent shaped side bearings may need a double encoder arm to be attached.
It's fun working out different scenerios before settling on a final design.
