A few quick remarks:
-the 39 mpix back sensor size is the same as the 22 mpix. Sensor of that size are 22, 39 and 50 mpix. Smaller sensors are 31, 40 and 50 (only cmos for 50).
-you can use film backs on the H2D and H3D cameras, that changed with the H3D-
II.
-22 mpix backs give a different rendering than 24x36 SLRs, but they are old, very sensitive to moire and service may be difficult
-31 and 40 mpix backs have twice the iso sensitivity as other backs, because they use micro-lenses.
-if you buy from the UK, you should check in other European countries. German dealers, for example, have good prices and will ship to the UK.
-if you buy from China, you should check in Hong Kong.
-if you want to use a flash in bright sunlight, indeed the central shutter of the H cameras is a big advantage over what the Pentax or your Nikon can do (even with dedicated flashes).
As a sample of UK and EU prices, you will find
here and English dealer offering an H3D-39 for £3500 (plus VAT), which is $5500. That is for back and camera. That dealer has good reputation but is a bit in the higher price range. You will find another H3D-39
here from an Austrian dealer for 4600€ (including VAT), which is $5800. That particular dealer will not ship to the UK, however.