Just finished rough assembly (ie, leaving off the shin/knee pads like BullDog said) and it's really the best GW/FW model I've ever built in terms of fit and finish.
I also magnetized my weapons arms -- too hard to transport him otherwise. I used 1/2" x 1/8" disc magnets from Amazing Magnets. They have just the right amount of grab. I tied 3/8" magnets, which would have been a fit with no fiddling in the arm joint (you'll see what I mean,) but they just didn't seem to have the right hold.
For the Tech Priest, I would go small. You could probably do 1/8" x 1/32" without any issues. I use that size for heavy weapon arms and the like on Termies, and they hold very well.