Origins pocket 1500/2000 with topper

I have been sleeping on a memory foam mattress - Silentnight 3 zone memory foam - since October 2012. However, it has been used on a sprung slatted frame and it has sagged in the middle. I have only recently learned from reading your articles that this is the wrong base for this mattress. In addition, I do find the mattress very warm.
I regularly suffer from a painful lower back and want to replace my mattress/bed with something much better. I weigh 15 stone (though I plan to reduce that to 13 stone in the next six months!) and I like a reasonably firm mattress, but I do like the soft feel of memory foam in the top layer.
I am looking at your Origins Pocket 1500 with the 2000 spring tension and the ottoman base. I am also considering a coolblue topper.
Do you think I will benefit from the topper? Am I correct to go for the 2000 spring tension?
Thanks for your advice