Bit the bullet..
Spent the last few weeks trying all manner of kit out, tried second hand phase one rigs, Nikon D800s and D800E, ended up going with a 645D:
Almost went for the 645Z, but it's nearly £7k and comes with loads of features I simply won't use.. I almost went for a D800E - which would have been the cheaper option, but having shot 5x4 for the last 5 years, the 3:2 aspect ratio of 35mm just totally turned me off, I missed the height in the frame, most of my shots I tend to work with big foregrounds and stuff - 3:2 is either too wide, or portraits are too skinny.
Managed to get one of the legendary old manual focus 35mm (26mm equiv) A lenses off ebay for £350, sharp as hell - easily as sharp as the P45+ rig and the hasselblad H4D rigs I tried.
Need to learn how to shoot digital now, the thing is like a toy compared to 5x4 rofl
Are you going to give wildlife a miss for now as I know one reason you like the idea of a D800E was wildlife potential.
You could also add a D7100 and the new 80-400mm as a cheaper wildlife addition to the pentax. You get a lot of camera for your money.