[chromo]

"my assumption is that maybe i need to place insulation and sheet metal straight in, with the help of the insulation the freezer already has?"

Depends on how your freezers made. If the insulation that is in it will withstand the heat of baking I would insulate the outside more, not the inside. If you add more to the inside you are making the oven smaller of course. You can add all you want on the outside anytime.
Is the inner liner metal? If plastic of course it has to come out anyway.

Also it looks like this one has some nice sturdy rails inside already to support shelfs. If it had the wire type shelves and you have them they might be perfect for parts. Sit parts on the shelves or hang parts on hooks under the shelves.

If the shelves are those plastic coated wire types, you can remove the plastic coating several ways, then you could powder coat them with High Temp powder and maybe be perfect. If chromed wire type they should be fine I think as is.

I can't tell enough about the freezer from here with just pictures so I have to guess at what it is or how it's made.
My guess and what I would do with this one first, use it as is after checking to see if it's solid foam or fiberglass insulation used in it.
I wish I had one like this if it's made like I think it might be.

If all else seems ok, metal liner, fiberglass, etc.. I would first put the oven elements in and get it HOT in an open area where any fumes would not be a problem, get it well over normal bake temps and see if it fumes or any problems. Once I knew that for sure then I would use as is if no problems.
Once you heat it up and hold the temps for about 20-30 minutes or so you'll get an idea how much heat you may lose from the case as is also then you'll have a better idea how much more insulation you might need.

I think I will go test some fiberglass now quick and post something about it an a few minutes.