Usually there is a plate around the inside of the front that is either screwed or riveted in. The inner liner will either be a set of interlocking plates or a single tub. Look for a thin line with silver sealant (caulk) or if it's all one piece inside. I've also seen models where the top comes off and the inner liner drops in that way. You just have to figure out how the built it as it varies by brand.
If I was converting that monster into a smoker, I would look at running multiple heating elements and a convection fan. With multiple elements you will be looking at a 220v setup. The average household outlet can only run a single 1500 element as they will max out at either 15 amps or 20 amps depending on what you have. With multiple elements you would get more even heating and be able to deal with that large volume of air (and cold meat).
Assuming you figure out how to get the insulation out and replace it, I would also consider making some sort of removable divider so that I could use 1/3 of it at a time for less than huge smokes. Either way, a single PID can trigger and control multiple SSR's to multiple heating elements.
Here is a thought, again assuming you replace the foam insulation, why not put in a permanent dividing wall on one side again cutting it into 1/3. Insulate this wall and you have storage shelves in the other areas for your accessories, grill gloves, smoke pellets, etc.... You should be able to run 1/3 of that cabinet just fine on a 1500 watt element and a standard 110v outlet. Like you said, you have it home now, might as well figure out a way to make it work.
If you only use it for sausage making and cold smokes you can probably leave the insulation alone. But anything over 190* I would suggest pulling and replacing it for safety.
Also I confess that I have a McCall proofer that needs the insulation pulled and replaced and I have been putting that off for a couple of years. So if there a "hangs head in disgust or shame" emoticon, I can use it also. Mine is more a matter of not having the space to properly pull the inner liner out of the cabinet (nor the time). I picked up the proofer when I was retired and a week later received confirmation they wanted me at another job and I've been there ever since (no time now). Paid $76 for the cabinet which is all stainless and has a recirculation blower system already built in.