So I have a shipping container 40 foot long and I was looking to insulate it and hook up electric to it and use it as my reloading  area also looking to add a window A/C unit.

So like to know if anyone has insulted one how did you do it or did you have to pay someone? Thank for the advice.

lived out of a conenx box for a short while while OCONUS.

We would use liquid nails and glue 1-2 inches of the pink foamular panels to the inside of the connex walls and 3-4 inches to the ceiling.  Leave the walls 3-4 inches short of the ceiling and then cut the ceiling panels full width so the ceiling edges rest on the walls for more support.  Next, we'd frame out the inside of the container loosely with 2x2 or 2x4 and then screw drywall or plywood to them.  you can cut more foam panels to fit between studs.  Obviously you'll want more strength wherever there are shelves.  Connex floors are usually hardwood, just toe the studs right into the floor.  We didn't worry about insulating floors because we always had boots or sandals/crocs on... just a rug or two.

R15-R25 for walls and R25-R40 for the ceiling will keep you comfortable.  I want to say 2" of Foamular foam is equal to R10?  We used to hook up an Alaska-Tent ECU to them for heating/cooling... but you'd be fine just a window AC unit (or single mini-split) for cooling and then a few 1500W coil heaters with circulation fans for heating.  Stay away from fuel and torpedo heaters because you'll get a headache from lack of circulation.  We were only living in 20ft conditioners, but I think the heating described would accommodate a full 40ft box.  We also insulated some by just hanging several layers of blankets (when that's all we could get) between and behind the studs and that worked OKish... You didn't die in the summer or freeze in the snow, but you had to dress smart too.

We would build out to leave both connex doors straight open (not fully open) so you can lay smoe plywood over top for a porch overhang. Then wall up the full opening with a framed wall (plywood and 2x4 and insulation) leaving room for a window and regular door.

most containers have breather vents in one more locations... cover them with metal screen, foil tape or metal or else you'll get bugs and bees all up in your area.

Years ago, I was out at someones property and they had used that spray in type insulation.
I know it really reduced the resonance (or Ringyness, I know not actually a word)...

However, I've also read people being worried about condensation and rust.

Not personal experience but have watched a bunch of shows with people using these as framework for a home. Spray foam seems the easiest and most often used.

I worked on a design for my brother a while back to clad and insulate the exterior of a Connex.

My input would be to cover the exterior of the connex and really consider venting as another issue that would have to be addressed.

Condensation is going to be a problem. As air warms it holds more moisture. Consider a cold soda pop can during the hot summer, and the beads of dew that for on the outside of the can.

if there isn't a heat source in winter to warm and a cooling source in the summer, insulation will only keep that box mild temp for a short period of time. Sooner or later, that connex will get to ambient temperatures.

I have specialized in exterior insulation and cladding for almost 20years, but I have yet to build a separate storage container that is separate from a dwellings existing HVAC system.

hopefully someone can be more help here than me

They 2x4 the walls and ceiling.
Insulated both inside and outside here.
Heat them with toyo stoves and even woodstoves.

Some are just spray foamed on the outside.

It's 105 degrees today in my neck of the world I does get cold here in the winter maybe below freezing a few days when it does. I'm mostly looking for insulation from heat.

I want to hook up power to it also and cut away a place for a window A/C unit.

Buy one that is a refer unit already insulated.

Potential fire or gas death trap.

https://www.thesafetymag.com/ca/topics/safety-and-ppe/worksafebc-warns-against-using-shipping-containers-for-other-purposes-/183992

What fire or gas comes from a window A/C? Like I said it's 105 degrees right now. Not in the mood for a nice fire. Winter it drops below freezing for a few days in December and January.

Seriously,I have electricity and natural gas in my house and it has yet to burn down. My conex has electric and a mini split with heat. I insulated with spray foam 3.5” on the walls and 5.5 on the cieling. It has no issues staying comfortable from 10 to 100 degrees. Colder than 10 and really the floor needs more insulation than the carpet pad

https://www.discovercontainers.com/5-methods-to-insulate-your-shipping-container-home/

If you have to build an interior stud wall, are you coming out that much ahead over just building a shed?

Only looking to build a reloading room! Don't need stud walls just insulation and an AC window unit!

I think the advantage is being able to close the ends and have it secured. You can also pick it up and move it. I’ve come back from chow and had my house moved.