Check tour heating coil by turning it on at night and then see if it glows a solid red. If it is even, then it probably the thermostat. They are easy to replace.
Another possibility since you opened it up, you may have kinked the capillary tube on the thermostat. If so, a replacement will be required. You can find one in the spare parts section.
As far as insulation, any high temp will work. I prefer a fiberglass or ceramic kiln insulation.
And if you want to make it a super smoker, you can get an Auber PID that has wifi to replace the analog thermostat so you can control the smoker with your phone, change temps on the fly etc. (I like my analog, so haven't done this.)
Another expense is to change the grates to stainless steel from the nickel plated originals. That is one thing I did do.