I know there are 50 threads about latex toppers, but what I'm interesting in knowing is if its worth even trying to use a topper for an already sagging mattress? And also, what ILD and type should I use? I prefer firmer support and do not want to sink into anything like memory foam and also don't want to sleep too hot, if that makes a difference.

If your mattress is old and sagging it won't help.  You'll still have the sagging.  However, if the reason behind the sagging is that the foam has crapped out but the springs are good, you could open the mattress, remove the old foam and use the topper to replace it.  That can work fine and save plenty of headache and money.
Kait