Good job trying to get things figured out before your chicks arrive! Mama Heating Pad is an excellent idea! You may also find this article helpful:
You Certainly Can Brood Chicks Outdoors
People usually make tractor coops/runs for
unsupervised time in the yard, their movability is to rotate the use of the ground. If you're going to supervise free range time, why not just let them loose in the yard?
I think you might find it much more economical to spend your money building a secure run
attached to the coop. That way they could stay outside all day (safely enclosed)... then get let out for some grassy time in the evenings. And if they're outside all day, the only space truly required
inside the coop is roost bars for sleeping... and egg boxes, which may lower your expense for overall coop size. Remember to make the nests lower that the roosts because chickens feel safer sleeping in the highest place possible, because you don't want them sleeping/pooping where the eggs are laid. Once the chickens are "homed" to the coop, they usually return on their own at dusk, without any bribes.
Also, that resin coop looks it would be pretty dark inside and l
aying hens require at least 14 hrs of daylight to produce eggs.
Ventilation is also super important... really important. Just their breathing and body heat creates moisture buildup, which can cause respiratory problems. Absolutely no need to insulate your coop as it would be useless with the required air flow present. Please read about the importance of
ventilation here.
The Rubbermaid type shed will need lots of modifications but many people do it. Click on
this search I did for links to some ideas to see what others have done. (Keep in mind though, not everyone has included enough ventilation)
How could I improve the ventilation of this shed to fit seven hens?
I think the best way would be to cut a large opening (or several) higher than the roosts will be, then cover those openings with hardware cloth. Drilling a few tiny holes will hardly do anything, and holes over 1/2" will have to be covered with HWC anyway.