So today I decided to take out the carpet in the boot, anyway couple of hours later pretty much taken out full interior apart from front seats

couple of patches on the floor that will need welding and a lot of rot around drivers side back window and on both sides on the back near rear surspension seams.(been told its common there) would put some pics up if I new how??
Anyway got it booked in to get rust sorted but when I gat it back I'm going to seal/spray all the floor and inside doors, boot etc and was woundering what would be best to use? I'm also going to soundproof it all before refitting carpets and was wanting a cheepish good sound proofing material?
Any advice would be good.
Thanks.