dont buy dynamat, it is a waste of money. use roof liner, grace under shield or other peel and stick products that contain bitumen. also use a high density rubber liner like epdm. if you want to. grace sticks to metal very well, has a good temp. range, insulates, as well it holds water very well, is self sealing, (so dont make pockets to trap water.) you can layer it. the one thing said by roofers is it sticks better to itself then anything. so it is very possible to layer it on as thick as you want or need. go and look this stuff up. grace ice and water shield.
when you spray foam the inner walls do it like an old packing style. you spray the foam into a bag that is already in place, tie it, then shove your cover in on top. you only get one chance, screw with the foam to much and it wont set right. do it this way so it is easy to remove if you need to get to some wires or replace a part of your car.
silicone all plastics and metal that touch or vibrate and you will have a more soundproof car than just dynamating the thing.