Hi Daisy320,
The first place I would start is post #1 here which has all the information, steps, and guidelines you will need to make good choices.
I’ve never managed to figure out how to convince anyone of anything other than presenting the facts as you see them and letting them make up their own mind 
As you will see though … I would always start locally anyway regardless of whether you end up purchasing locally or online and would only look online if the difference in quality and value was large enough to justify the additional risk of an online order… While the membership here is certainly among the best of the best in terms of quality and value … they are only a small group so far and there are many sources of great quality/value across the country that are not members here.
I would be very cautious about using another mattress as your “target”. In terms of quality and value I wouldn’t want an equivalent to the iComfort anyway and in terms of “feel” which is very subjective and relative to the body type, sleeping style, preferences, and perceptions of each person … the only way to find a “feel” equivalent is with side by side testing in real time. As you can see in post #2 here … I would measure every mattress you test against a common set of standards … not another mattress. The time you spend testing mattresses like the iComfort and other major brands are mostly wasted because they are not worth buying in terms of quality and value and they can’t be “duplicated” outside of their “subjective feel” even if someone wanted to because they don’t disclose the details or quality information about what is in their mattress.
As you can read here (and is also in the first post I linked) … warranties are about marketing and have little to do with how long a mattress will last or when it will need to be replaced. Warranties only cover defects and the loss of comfort ans support that comes from foam softening andis the main reason a mattress needs to be replaced isn’t covered by a warranty.
Most ofthe information you will be told about sleeping temperature is also about marketing and is more misleading than anything else. you can read more about all the different factors involved in the sleeping temperature of a mattress in post #2 here and post #29 here. Most of what you will hear from mainstream stores is about “selling you” rather than “educating you”.
Some of the better options and possibilities I’m aware of in the Gainesville area are listed in post #2 here.
Phoenix