1. I used to have a K100D and now have a K-3. From what I remember, the K100D's flash housing protrudes very far out from the camera. I actually used to pick it up from the flash housing (probably a bad idea but oh well). The K-3 protrusion is not as large nor as low.
VS
2. The K-3 viewfinder has a greater magnification factor than both the K100D and the K-5. It is also brighter than both. But to tell you the truth, I'd focus my MF lenses with live view and focus peaking. The K-5 has live view but not focus peaking. IMO, focus peaking is the best thing that has ever happened to MF lenses.