I actually have one of these that has been in use for quite some time. As mentioned above, it is very low-tech, but not stone-age.
The size of the metal doesn't really concern me, it looks much thicker in person.
Mine is at least 25 years old, was outside (in use, exposed to the elements) for at least 4 of those years, and while showing her age, is still perfectly functional. It has since been moved into the (detached) garage, where it heats a 24x24 work shop (walls insulated but no ceiling, 2 garage doors), and a 24x14 apartment / guest area. I did have to replace the damper control motor (replaced it with a BELIMO from ebay). It came second hand to me with a Honeywell aqua-stat installed to control a set of relays, which controls the burner, pump, furnace blower motor, etc.
I have added firebrick & a grate inside, and an external flue-gas heat exchanger in the style of a 55 gallon drum with some baffles (a lot of heat goes up the stack. it's easy to get flu temps over 1000* within 18" of the outlet without even trying). It smokes quite a bit when it cycles, but with the firebrick, it is much better once it gets heated up. Once it has burned for about 5-10 minutes, the smoke dissipates to near-zero. I'm working on an arduino control system at the moment, just got the sensors yesterday. I've also acquired some (rock-wool?) insulation to wrap the girl in so she can stay warm...
I refer to it as a highly polished turd...lol.. but it's not so bad. The whole system (about 30' one-way of 1" copper to an 80KBTU HX) holds approx. 15 gallons of water @ 12psi. With a full load of good oak, on a 10-15* night, I typically get about 6-7 hours burn time with the thermostat set at 67. It keeps both sides of the garage toasty warm, even though the thermostat is in the insulated apartment (the barrel heat exchanger made a huge difference in the workshop). I do wish it had a larger firebox, but for what it is, I have got it working pretty darn well.
One good thing that has come from it - I found this forum, and have been occupied for the last 3 years now reading, revising, and otherwise tinkering with this darned thing. It has been a lot of fun to piece it together and take it apart 3 times a year, making improvements...lol.
If you can get it for around $500 or less it seems like a good deal to me. Just be ready to do a little tinkering... If you want to see pics of my Frankenstein, pm me your email addy.