Replacing a Sealy Posturepaedic with a split tension mattress

Hi John & Ryan,
I'm looking to buy a high quality std double or kingsize mattress/bed and I've found your web site to be a great help explaining the fetures and pros and cons involved in making a short list and final selection.
I'm 6ft and just under 13st while my wife is maybe 5ft6 and usually under 8st. We are both in our early-mid 60s and have typical aches and pains in backs, shoulders and hips especially after a night on our (20 year ?) old Sealy Posturepaedic mattress. Our budget could stretch to £2500.
Our current base is an even older Myers 2-piece unsprung drawer divan which seems to have wooden slats running lengthwise, covered in a thin layer of foam and material. So, my first question is whether we should go for a more expensive mattress and retain the existing base, or split our budget to include a new base and if so then what would be your recommendation for the relative importance and budget split between mattress and base ? If we keep our old base, should we consider using some MDF board similar to your suggestions for a slatted base, and if so would you recommend using something like a Vispring bedstead mattress pad between the MDF and the mattress ?
We don't want a memory foam or laytex mattress and are pretty much decided on a pocket sprung mattress. Given our weights, the suggested spring tension would be medium for myself and soft for my wife. So my second question is would a single tension (medium or soft ?) be suitable, or should we be looking for either a dual-tension mattress or a zip & link ? I would imagine the restrictions on ability to turn/rotate would be similar for dual tension and zip & link, unless the latter had zips on multiple edges of each mattress. Although you imply this might be an extra cost option, I can't find any specific details.
What would be the additional cost for dual tension ? I would also imagine that zip & link will be more expensive than the equivalent one piece, but how much more would be resonable ?
At the moment, my short (?) list contains your own Artisan mattresses, the John Lewis own brand Natural Collection and the Vispring mattresses made for JL. Would you agree that these 3 ranges have very similar offerings, with the JL Naturals perhaps lying between the other 2 in terms of price points for similar specs, but what are your views on the relative merits.
With regards to specifications, JL provide details of the spring counts and the various upholstery layers but do not give any GSM figures. I've also looked at the Vispring web site and they only give the same details as JL, and I can't find any GSM figures contrary to what you suggest in your replies to other comments. Your Mattress Comparison page has very detailed GSM figures for Vispring so could you confirm where you obtained those figures.
Sorry for the lengthy post. Any additional advice would be most welcome.
Thanks in anticipation.