Hi Sunshine4Me,
I think the first place I would start is post #1 here and the information it links to.
It seems to me that based on your description and budget that you are probably looking for a latex hybrid with latex in the comfort layers and either polyfoam or an innerspring in the support layers (an innerspring will likely be a bit more).
There are many different versions of these so local testing with the help and guidance of a knowledgeable salesperson or consultant will help you match a mattress with the “right” layering and firmness combination to your specific needs and preferences.
There are many factors involved in the sleeping temperature of a mattress but latex is the most breathable of the foam categories and you can also help this by making sure that the other factors and layers are in the direction of more ventilation and breathability as well. There is more about this in post #2 here and in post #29 here
Once you have gathered some basic information about mattresses and you know how to eliminate the worst choices (from the post I Iinked) … then it’s time to find the better local manufacturers and sleep shops in your area and begin to do some mattress testing.
If you let me know your city or zip … I’d be happy to let you know about any better options or possibilities I’m aware of in your area.
The most important part in making a good quality/value choice is to always make sure that you deal with manufacturers or retailers that will provide you with good information about the quality details of every layer in any mattress you are considering so you can make meaningful comparisons with other mattresses and that also have the knowledge and experience to help you make the most suitable choices for your body type, sleeping positions, and preferences.
Feel free to post with any questions you may have along the way.
Phoenix