Mattresses for varying weights

Hi,
Firstly, what a fantastic website. I've learnt more on here about mattresses in half an hour than in have in a lifetime. Great work!
We need to replace our mattress, but both myself and my partner vary in weights quite dramatically, she being 9 stone ish and 5ft 4, me being nearly 20 and 6ft 4. Ideally. From what i can gather from your site, a Zip and Link (ideally king size) would be the best solution. We are, however restricted by budget £600-£800, thus are looking for the best all round compromise. Would an Origins 1500 with topper suit, or should we be looking at something else?