Where do you buy your consumables for your printer? The supplier should be able to help with what is best for this sort of work, what is it you are printing and at what resolution?
Reason I ask is because things like this make a difference, the other thing I would ask is why mount to 5mm foamex? Is there a reason for this or is it because you dont know any better?
Sorry if that sounds a bit blunt it wasn’t meant to but there are normally a lot of factors to be taken in to account when doing exhibition graphics and although I am not an expert on this subject have looked into doing it on a larger scale.
As far as printing goes though, I would think paper would be suitable for something like this, as Jayne says use a good quallity paper and try to stay away from gloss papers. You will need a mounting film like printmount one or similar and if over laminating use a matt laminate. This is mainly to cut down glare from exhibition lights.
Idealy both the mounting film and the laminate should be applied with a laminator but I have applied both by hand on the bench at large sizes and got good results. If you already print to vinyl this would probably be suitable but only a test print will tell, just overlaminate the vinyl as you probably do already.