When buying a bedstead with a mattress, use a mattress pad over the wooden bedstead with a mattress, use a mattress pad over the wooden bedstead slats.
The foam can simply be added to the top of your mattress before you put your sheets or mattress pad on the bed.
Think about the mattress - Before you start tweaking the plans to add a few extra inches here and there, consider the fact that the textiles, like sheets, blankets, and the mattress pad will probably have to be standard manufactured goods.
If you suspect your mattress pre-dates indoor plumbing, an egg crate mattress or mattress pad can make all the difference.
To complete the bedding, you will also need to find a fitted mattress pad and potentially a waterproof liner.
