The D-number tells you the speed/grain of the film, I know that. It is also very high contrast.
Of all the Structurix D-numbers, D6W is the one you want. D4W is the next easiest to work with. These have the closest to pictoral contrast with reasonable development, and D4 is roughly 100 ISO speed.
D numbers - ISO speeds:
D2 ISO 32
D3 ISO 64
D4 ISO 100
D5 ISO 200
D6 ISO 200
D7 ISO 320
D8 ISO 400
Yep, D5 and D6 are both ISO C4 class, so both are about 200 ISO speed.