Hi Bjc1988,
The first place I would start your research is the tutorial post here which has all the basic information, steps, and guidelines you will need to make the best possible choices.
When it comes to Amerisleep I think my recent reply and the links in post #2 here would apply here as well. You have many good memory foam options available to you and while Amerisleep may be better value than many mainstream choices … they wouldn’t be my first choice with some of the options you have available to you.
There may also be some good options available to you locally and if you let me know your city or zip code I’d be happy to let you know of any I’m aware of in your area.
A memory foam mattress usually does best on a rigid non flexing support surface which could be a platform bed or foundation with either a solid surface, slats, or a wire grid (my preference is slats so that there is even solid support and some airflow underneath the mattress). If you currently have a box spring that flexes or that is sagging I have no idea why they would suggest you continue to use it. That seems very odd to me.
Phoenix