Thank you kindly gents. Sendak, the battlesuits are space marines kitted with Pig Iron militia helmets, an idea from WB Kurgan.
The machine guns were indeed supposed to be in cupolas, but I couldn't find anything that worked well enough.
The big guns are supposed to be:
2 x 280mm 54.5 SK C/34
1 x 128mm KwK 44 L/55
If I had better modeling skills I would have seen about some scratchbuilding -- especially for the turret (looks nothing like how its supposed to be).
Forgot the railings -- had originally been planned. The ladders are a great idea. Need to put some in!
Side guns would be nice (as would a cut away), but I'm still toying with how far to depart from the actual idea they had for the vehicle (given that the model isn't the best for the job in any case).