Can you post pics of your deck, especially close ups of the structural framing?
This is the best way to gauge what shape your deck's structural framing is in.
What is "way off the ground - 8', 20'?"
If the posts are rotten, chances are good that the rest of the structural framing is in need of replacement.
Are your existing deck boards screwed or nailed down?
If screwed, you can try unscrewing them. Many of the screws will probably break, leaving the shank in the deck board. You'll need a heavy duty pry bar to pull up the old boards.
You could try this: http://www.deckwrecker.com/
Are you physically capable of prying the old boards up?
If nailed, you'll still need to pry up the old boards. Nails pull out easier than screws but it's still a challenge. Start at one end of the board and pry it up until the nails are free. Work your way down the board.
Are you planning on replacing your railing too?
To answer your other question, No, composite wood can not be used for structural purposes.
Know this, a deck that is "way off the ground" can kill people
if put together improperly. I'm not saying you can't do this, just know what could happen if you do an improper job.
There is a lot
more to building a deck, especially an elevated deck, than merely "sawing and nailing." It is not unrealistic to think that you could replace deck boards - it might be unrealistic to think that the finished job will look great.