Chris, I think our attitudes likely are similar. The more you describe your style, the more I think you would get on well with the 645D. Sorry, not trying to sell you on this system.
I also don't want to underplay the potential challenge of manual focus and need for good glass on this camera, not that you sound like you need the caveats. The finder is big & bright but not as well suited for manual focus as the last generation of film bodies. And the 645D has less depth of field than a 35mm format body, of course combined with high resolution. So the acuity of the system punishes focus errors or lens issues. But if you are well suited to the manual style of shooting and willing to work with it, I don't see that you can't make it work. (I think Pentax obviously is depending on people being willing & able to do that, because there isn't a flood of lens options otherwise.)
I don't have experience with the old school Takumar lenses but at those prices there's likely no financial downside to try them out. I'd think the Takumar 105mm f/2.4 could be useful, as an example, if you desire it for shooting more wide open. But Paul's comments have the ring of experience -- don't overly judge the performance of the entire system by them if those lenses don't hold up. Great glass is very rewarding on this system, and vice versa. (See the recent thread on Luminous Landscape where a gentleman posts about his issues with the new 55mm.)
I got my 67-to-645 adapter from my local Pentax dealer here in Calgary, they simply ordered it up like normal from Pentax. I don't know if this means the adapter is still in production, or that there's simply a small supply floating around as "new in box" old stock. My dealer seems to be able to get hold of a fair amount of 645 stuff, but then they are one of the key pro dealers in Canada.
KEH also has been mentioned, that's an important source, at least for 67 kit now that the 645 inventory has been largely plundered. I've ordered from them for years, and have recently made a few new acquisitions from them for the 645D...