Bed construction for pocket sprung mattress

Hi, I've just found your website and oops just what I need. I'm planning to build a bed, having had solid and divan beds in the past. As I'm going to buy a pocket sprung mattress I was concerned about the sizing and spacing of the slats. Also reading your suggestion of boarding over inadequate slats, I was then thinking of a solid base, but, then concerned about ventilation. Can you advise re the above. Also wondering what you think about having two separate mattresses of differing firmness I've zip and lock. Do you sell these?
May thanks