Where are you trying to source these from? Sennheiser charge nothing like that amount.
The Orpheus is brighter than the SR-007, but I expect the majority of people have not heard the two side by side
There's no right answer, build them and see if you like them! Obviously a lot of people like sr-007. A lot of people would consider my orpheus clones too bright (which by the way are 84 x 122mm active area), but they're exactly what I want and are quite possibly the best headphones out there for me, but not for a lot of people. The greatest thing of making your own headphones is you get to tune them to what you want, which is difficult to do without experimentation.
I did once make 56x82mm headphones, and I'd say they were definitely too small. Pretty much impossible to get any bass out of them. 70x105 is certainly an interesting size to try.