Difference between Mattresses of Different types?

Hi,
We are looking to buy a king size mattress for our master bedroom and have a few queries / questions that we would like some input on from your side please.
We want a medium/firm mattress ideally but something that retains its firmness over age, so should we be better off going for a firm mattress now with the assumption that it will become less firm in 1-2 years time? What is the normal firmness retention for your mattresses?
Ideally we would like to avoid any options that would become very hot or uncomfortable during summers / night due to poor heat dissipation.
My wife is a light sleeper so what would be the best option to ensure that if I did turn in bed, it would not disturb her to break her sleep? I am about 16 stone and my wife is 10 stone so what would be a good option for us?
Also we have a wooden slat base for our king bed, so would the weight of the mattress cause sag / damage to the slats and should we look at the lighter weight options with pocket springs instead of the heavier Artisan options?
For the delivery, we live in an apartment on the first floor of building, without a lift for access, so would the delivery company be okay to deliver to the property?
Based on the above questions in our minds, we have narrowed the choices down to the Origin Pocket 1500 / Origin Latex 1500 / Artisan 1500 Mattresses from the website. Ignoring the price difference, which option will be best suited for us to meet as many of the requirements listed above? I appreciate it's not possible to tick all the boxes but if we can tick majority of them, that would be appreciated.
And what is the current delivery lead time for the 3 options listed above? We need a mattress in 2 weeks time so would this be possible or not?