Don't over think it, it's used to start seeds not grow plants on that's why it's called a seed starting mat and not a growing mat so just grab a cheap one and go with it. Remove the flats after the seeds pop and you'll never have to worry about burning your plants.
I have the plain Jane he he!
iPower Seedling Heat Mat without a t-stat and they have been working as described=10-20° above ambient temperature. They withstood the moisture in the greenhouse for 2 years so for the price I can't complain.
You can also get them from
Prime if you have it.