I have a 6X7 with a 105, 135, and 200 I used to use the 105 all the time and the 200 once in a great while, now the 135 is almost all I ever use. The 135 can get you in a lot closer and allow you to fill the frame with your subject.
As for the mirror shake that you hear about, here is MHO on it, I am a lousy photographer, I take crappy crooked images and I have taken hundreds of photos with my 6x7 in both color and B&W. I have never had my 6x7 on a tripod for anything but a long exposure night shot, and I think I have done 20 of those ever. In the literary hundreds of handheld shots I have made with this camera I have had one blurred image and that was because I was shooting in low light and failed to note the shutter speed.
You might want the leaf shutter 90, but you don't really need it. As for the flash sync speed thing, I don't find it to be that huge of a problem, at least it hasn't been for me.