Before I decided to simply replace the decking boards I would want to know two things. First, what kind of condition is the ledger board (attaches deck to house). Does it appear solid with no rot? Was it attached properly with lag bolts and not just simply nailed to the house? Is it properly flashed to keep water from seeping into the house?
Second, are the posts mounted to a proper concrete footer? You mention that the outside edge of the deck appears to be a few inches lower. The deck may have been built this way to shed water or it may be because the posts are not supported properly and have settled.
Other items to consider is whether the deck joists are spaced properly, if the deck is cantilevered are the proper span distances adhered to, and so on. I would recommend a good deck building book to compare your existing deck against a well-built new deck then decide what needs done.