I'm the self-appointed expert at getting it halfway done.
See my post for reasons why.
Replacing carpet with Allure. Ouch!
Most folks use 1/4" luan for a overlayment, I think. My question would be if that would be thick enough to compensate for the sagging issues. And then, if you go thicker than that, will it interfere with your slide when you retract it? Wiser souls than I will comment, I'm sure. (Cue Ernie Ekberg....)
As far as what type of materials to use, it depends what you want. Replacing the carpet with new carpet will be the less labor intensive, IMHO. If you want more of a tile/wood look, then there are several options. Allure has been used by several and it worked well for them. However, the manufacturer of Allure does not recommend using it in temperature extremes. Several other folks here have used it and had some issues. I decided not to use Allure, and went with a glue down LVT/carpet combo.
i learned that you need to get as best a plan as possible before you start, but be prepared to be flexible. For me, figuring out how to handle the slides was the biggest issue.
There are some wonderful resources on this site. They were very helpful to me, and I'm sure they will advise you, too.
Pics are always fun, BTW.