PSI and pins and needles when I sleep

We have an old matress on a cheap bed size is king size.
I am 15stone, other half is 9 stone.
She suffers psi and needles when sleep with no foam topper,
I like a firm matress
I have abad back
We suffer roll together in our bed.
I sleep best in the spare room on another bed, she sleeps best in our bed when I’m not there !
Goal – to sleep in the same bed and both have a good night sleep.
I’m thinking we go for a bigger bed so we can both sprawl, so a super king, a sturdy wooden one with a fully doing slatted base I have seen that we like.
Next for the matress, I would love a firm one, though obviously don’t want my girlfriend to suffer pins and needles, so thinking a firm matress with a topper again, but a super king topper will be expensive for a good one on top of a matress.
I have seen these mattresses from silent night, the geltek ones, bit there is a springs/” title=”pocket spring”>pocket spring ones and miracoil ones,which is best ? I also don’t know from reading them if the 1000.1350.1850 pocket spring ones or micro spring one at a lower price would be better, I don’t want to get one too soft that will aggravate my back, what is better pocket spring or miracoil.
I like the idea of the gelteks as I get pretty warm with the topper on, I have slept in memory foam matress before and it was awful for my back, so as far as I’m aware too soft is bad for me !
Budget is anything up to £800.
I look forward to your thoughts, my logic is saying firm matress with a topper still, oh and ps am happy to get dome mdf to put on slats of bed to make flat surface (does that make the matress firmer or softer though ?)