Hi Tyson,
I’ll give you some very quick and simple answers.
Toppers: Look in post #2 here and the posts It links to as a guide for selecting a topper, including memory foam.
1.5": That’s not a normal thickness, so you may want to look at some of the memory foam suppliers in the component post here who may be able to fabricate that thickness for you in case you can’t find a 1.5" elsewhere.
Sleeping cool: All foam are an insulator to an extent (especially memory foam), and regarding cooling memory foam claims, you can read more about phase change materials in post #9 here and at the end of post #4 here, and you can read more about the various different types of gel foams in post #2 here. In general terms gel foams will tend to have a temporary effect on temperature while you are first going to sleep until temperatures equalize but have less effect on temperature regulation throughout the course of the night.
I hope that’s helpful and not too complicated! B)
Phoenix