Looks like my old New Braunfels Black Diamond, affectionately dubbed NBBD by a whole generation of BBQers.
I know there's something satisfying about heavy gauge steel. But 3/32" works just fine. It won't retain the heat as well, and a bit more of the heat may pass through to the outside, but it's thick enough so the rust can be scrubbed off many times.
I made several changes to mine, which came to me with no cooking grates. Besides making a pair of grates, I added a baffle and tuning plates. I removed the stack and sealed the hole. I made another 4" stack which came straight out the center of the end away from the firebox, then turned 90 degrees and went up. Also, I did a lot of work to seal up the many air gaps. I also fitted it with a propane burner going into the inner end of the firebox down low. I could use that as a log lighter or I could just cook with the propane adding bits of wood for flavor. I also made a charcoal basket and lined the bottom of the firebox with firebrick. I used parchment paper and hi-temp RTV to form gaskets around the main cooker doors to seal those up. I don't think I added one or more thermometers. In those days I used one of those digital probes and I would stick one through a potato and leave it in the oven. I could read it from the outside.
That wound up a pretty nice cooker. I sold it to a guy who wanted to get into BBQ competitions. So I don't think you wasted your time. Enjoy it while you own it. I'm sure someday you will sell it and recoup all or mostly all of your investment, and then you can try a different type. The world is full of cooker designs. The NBBD was a classic.
Yours could also be a Silver Smoker or a Hondo or a Luckenbach.
seattlepitboss