Remove 2x12 ledger board, see if you can get flashing slid up under siding and around door (google Z flashing)
Replace the 2x12, use treated. I assume it's bolting to wood joist plate and not block etc. Use a fuck ton of lag bolts if you can't get to it from the inside, try to hit the interior joists (you can google ledger board attachment as well, as long as you're tying into wood it shouldn't be too worrisome). I think i read like 80% of decks fail because of ledger board attachment failure, so keep that in mind.
Use joist hangers on the ledger board to attach new joists, use screws and use all holes of the hangers.
If the deck is more than 4 or 5 feet off the ground add some angle braces to the posts.
Regarding interior water damage and rot I would fix what I run into, but I wouldn't bust out drywall that looks fine looking for it. I'm assuming this has been a long time coming, if it's not and you built this house or remodeled within a few years, you need to do some serious shit.