ViSpring Elite + topper

Hi, I'm looking at the ViSpring Elite mattress (Tried to find on your site but no Elite model comparison).
The reason being is that my partner want's to try a mattress out in a store before buying. The only one within our price range that seems well enough is the Elite, although it doesn't leave much on my budget for a mattress topper - I will outstretch the £1000 if it seems warrented.
I've read that you should keep the similar materials in regards to topper and mattress, but I've tried a 100% duck down mattress topper from Soak&Sleep(when they were called duvetpillowcompany) and although it felt like laying on a cloud, I felt it was very warm and ended up not sleeping on it and in fact giving it away.
I do like the look of your coolblue topper, but I can't help but think if I spend around £1000 on a mattress, what's the point if I won't feel the benefit of it and only really feel the topper?
Will this topper be suitable with a Vispring Elite
If new beds sag from the materials, why not just buy a topper to begin with if it will be susceptible from a sagging mattress from the get go?
Also, for my sake to which I will try to convince by partner into buying one of your mattresses - What is a comparable mattress to the Elite? I don't think I'm getting much bang for buck when they only have a depth of 21cm and wish to know if you supply one similar for a price guide into what I could be getting from you.
Many thanks
Jonathan