Hi sydrt,
I don’t have personal experience with a Murphy Bed but post #23 here and post #64 here may be helpful. I would tend to focus on an innerspring mattress (with helicals joining the coils rather than a pocket coil) or a mattress that has a firm and fairly “stiff” polyfoam support core but an innerspring would most likely be the better choice.
Other than that … I would follow the same steps in the tutorial post here as I would for any other mattress that wasn’t in a Murphy bed to choose a mattress that was the best match for you in terms of PPP, durability, and all the other parts of your personal value equation that are most important to you.
Phoenix