I've really been meaning to make a post with all the part numbers for the IDI heads, but I've discovered that the VW part numbers do not match the casting numbers. This is due to the fact that the ETKA lists part numbers for complete heads (valves and all), and not the part numbers for the bare heads (i.e. casting numbers). Thus, while there may be different part numbers for complete heads, there may in fact be no difference in the number cast on the heads. This is likely due to the fact that the only difference between the IDI NA and turbo heads seems to lay in the valves. The Turbo uses sodium-filled exhaust valves to deal with the added heat, the NA does not.
I'm fairly confident that this is the only difference. As an example, the TD head on my 1982 Quantum has a casting number of 068 103 373 F. I pulled the original head off of a 1985 NA Golf with mechanical lifters and it had the same casting number.
Now, there are sepculations about different head material, different seats, etc, but every head specialist with experience with VW diesel heads that I've ever talked to claims that there is no difference in the head with respect to material in either the casting or the seats. So, take that as you will.
I'm happy to be wrong, but that is how I see it based on my research. If someone else here has some sound evidence to the contrary, then I'd be happy to hear it and revise my position.
As per buying a new head. Be advised that many of the "new" heads for sale out there, as many are made in China with inferior metal. The brand that comes to mind is AMC. I've got one of their heads on a motor that I'll be junking soon. Mind you, this head was put on a motor that had overheated before, and this head was put on as a replacement. I pulled one of the cam caps, and it looked like a lava flow. It almost appeared like the metal had flaked off, and then re-formed within the journal cap. I've never seen anything like it. Speaking with people who rework diesel heads, they say if forced to weld one of those heads, the rods blow through the aluminum like butter: leaving large molten holes. All-in-all, I'd steer clear of any head supplier who wont' tell you the manufacturer's name.
Basically, I'd only trust OEM, or Kolbenschmidt. And they aren't cheap. It just goes to that old saying: if it sounds too good to be true, it probably is.