The recommended order of business is to tackle air infiltration before insulation. There is quite a bit of information available on the internet for this topic, such as:

http://www.simplehomerepairs.com/Weatherproofing.html

This means caulk and weatherstrip. Especially as you plan to heat your shop from "cold" prior to use with a convection heater (heating the air, not the surfaces); if you don't keep the cold air from continuing to come in you will end up with a hot head and freezing cold feet!

In addition, insulate the ceiling as recommended here. Insulating walls is much more work, so save that for another year.

I suspect a reason you are getting your shop up to around 52 fast, then having trouble above that is that all your heat is rising above you and shutting off the heater (mounted high with an internal thermostat, I suspect). A remote thermostat should help with this, mounted at the level of the people.