Hi robr,
It would be difficult to emulate the Prana because it’s a very different construction and has 3" of polyfoam over the latex. I’m not sure which of the Ultimate Dreams mattresses you are looking at but probably the closest you could come would be to “rate” the Prana you slept on a comfort scale from 1 - 10 (Ultra firm sleeping on the floor to ultra plush sleeping on a cloud) and then talk to Chuck to help you make an approximate match to your assessment as accurately as the two of you can figure out. I don’t think Jordans provides enough information about the comfort specs of the Prana to be any more specific than that and even if they did the designs are probably too different to really match them. In other words you may need to match your subjective perceptions of the relative firmness of the Prana comfort layers with the firmness options they have available. Depending on your confidence that you can match the feel of the Prana … you may also want to consider either their Eurotop on Amazon or one of the mattresses on their main site which has exchangeable layers so you can exchange the top layer for something firmer or softer if you need to.
As you mention the other option would be a topper and if you do decide to go in this direction I would first order the mattress as close as possible to your preference and then let your actual experience on the mattress help you decide how far away you are from your “ideal” and then choose the topper that you think may be best using the topper guidelines here and here.
I would always deal with only one variable at a time and let your experience on the mattress help you make your best topper choice.
Phoenix