Personally, I would get aftermarket hands and arms from Anvil Industry, Mad Robot, any manufacturer that makes aftermarket bits will have kits with non-weapon hands and arms.
As far as seated models are concerned, there are plenty of WW2 kits with seated crews that have separate bodies and legs. I'm pretty sure Mad Robot makes seated legs as well. Anvil industry as well, but I'm fairly certain they are designed for use with their heavy weapons kits.