Soft mattress for the over 11stone (just)

My partner has a connective tissue disorder and after trying some beds prefers a soft Vispring Regal Superb. We have a limited budget and wondered is it OK to have a softer mattress and weigh 11st 2lbs. We have an existing divan - is it better to buy a complete new mattress and divan which is cheaper or use our existing divan and have a more expensive mattress?