The Equilibar regulator can be operated at very low exhaust pressures such as those you mentioned with no problems [and in near vacuum]. In fact we pride ourselves on the performance at low pressures. Do you have a pressure that you wish to maintain inside the suit/helmet? It would seem that somewhere close to 1 atm would be needed to sustain lung function. We also need to understand the flow rate and our quick experiment suggests that about 10scfm seems to be appropriate. Of course that rate is based on subject not performing heavy work--our tester was only standing and exhaling into a flow meter.
If we have your best estimate of those those variables we can give your an estimate for our appropriate standard product. And, from there we can explore the rest of the challenges
To clarify things, I replied to them with some information, and the following crude sketch of my pressure suit; we'll see what happens next. Immediate plans, though, are to suspend this project as I need to get cracking on preparations for Alaska this winter!