Choices, choices... On the original 645 you're really limited to either the 645 A55/2.8 or a 67 55mm (there are three versions) - the 645 D-FA 55 doesn't have an aperture ring, limiting it to Program or Tv modes.
As a 67 shooter myself, I'd probably go for the late 67 55/4 to give me more options. That said, I have the D-FA which is my preference for my 645D (the posts above suggest I got it for a bargain).
As for keeping it consistent, Pentax 6x7 glass comes in four main flavours - Super Takumar 6x7, Super-Multi-Coated 6x7 (both mostly metal barrelled with gorgeous Takumar fluting) and later SMC 6x7 and lastly SMC 67 (progressively more plastic and rubber). The formulas in many of these lenses are identical across the decades, however the 55, 75, 90, 200 & 300 came in later versions with notable improvements.
Consistency may depend on which version of the 105 you've got, the 'early' 6x7 55mm f/3.5 might fit in better with an earlier 6x7 Takumar style 105, and also gets good ratings (but takes huge filters). Check out the 6x7 lens reviews:
Pentax 67 Wide-Angle Primes - Lens Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database