What about.....

I'm thinking of making a hobby of making one of these into a Viv. I found these on my local craigslit and thought it would be beautiful. They're just normal store display cases, but have sooo much potential. I know I would need to put a lip inside for the substrate and have someone put air holes in it. It will take a while, but was wondering what I would do about heating.

I think that's a really good idea if you can work it out. You could make a real attractive display viv with either of them.
I've seen references here about placing Ultratherm mats in the viv. I cut out part of the bottom of my viv and put in a lexan plate to put the stick-on UTHs under it. It seems to me you would use less energy for heat if you had the heater in the tank instead of under it.

i would advise against heating it with a small campfire

nice find however

I just sold my 2 display cases....but this thought crossed my mind SEVERAL times! It would be hard to make it waterproof, but if it's cheap enough you should be able to get a few years out of it before it is ruined by spilled water bowls, etc.

That would make a great display cage after some work done...
Buy silicone and waterproof all the joints. Like Drew said, if they are cheap enough, you may not even care about waterproofing it. Install a leap for the substrate as you mentioned. As far as ventilation, that could be done on the doors in the back. As far as heating, I use a UTH inside my viv... I just stuck aluminum foil on the sticky part...
Great project!