Hi maybeddoll,
First of all … congratulations on your new mattress 
I think you made a good quality/value choice and I’m looking forward to your feedback when you receive it.
Post #2 here includes links to the better forum posts and topics about children and mattresses that should help you choose the types of materials and mattresses that are the best “match” for your criteria.
Your budget is on the low side but if you are looking for a twin then that would help a bit (larger sizes are more costly). Post #4 here includes some of the better online options I’m aware of that are in a lower budget range although I would tend to avoid any of the memory foam options that are listed for younger children.
Phoenix