I am in the process of finishing a new 24 x 24 x 12 red iron building and could use some advice. I have not decided on a brand yet but I have read through a lot of threads over the last 2 weeks. What size should I get with a 12ft ceiling? I am looking at a 18000btu or should I go 24000btu? It has standard 3" vinyl insulation and garage door is insulated. I am located in Oklahoma. It will all the time when weather gets hot and when cold.