You'd need at least 2 4X8 sheets to do a MK III suit. You might be able to squeeze a MK V out of one 4X8 sheet. The Mk III has so many surfaces you'll blow through plastic. The Mk V legs and arms consist of (basically) a lot of rectangles. You can cut out the rectangles strategically to reduce wasted plastic. With the organic surfaces of the MK III, you have more wasted plastic, so it seems to take more. I'll do a test later to verify this for you.
With sintra you can be much more precise than the plastic from the trash can. With sintra you heat it, bend it, and hold it until it cools. It will stay in the position you held it while cooling. With the trashcan plastic it gives just a bit, so the position it cools in is
almost where it will stay, but not exactly. It also is much softer than sintra. It works, but it's not as precise and I don't think you'd be able to put bondo over it like you can sintra.
So for "building" I'd use sintra,polystyrene,PVC, etc. If you just need a sheet pf plastic that needs to be slightly shaped but not hold a body filler, I'd use the trashcan plastic.
I've only worked with 3mm sintra and 6mm sintre (never again). I use a
razor knife to cut the plastic and I refresh the blade very often.
I use multiple shallow cuts to cut the plastic.
Hope this helps.