Hi Dave. I totally agree with what has been said. My two cents worth...
I lusted after an alloy bodied Abarth for many years (since birth I think) and eventually bought a Record Monza that needed body restoration, but was pretty complete, except for the engine which came disassembled. It even had the correct alloy bumperettes which are often missing. But.... There was bad corrosion where the aluminum meets the 600 steel body. I kept it for several years, pondering how to restore it. I had a knowledgable restorer look at it (he does world class restorations to Mercedes Gullwings, Ferraris and other exotics) and he said the body restoration would cost around $40,000 and that was about 10 years ago. I had a good 600 shell that I was going to use, and had planned to remove the complete aluminum RM shell and transfer it over. The steel in the RM was REALLY bad from the galvanic reaction/corrosion.
In the end, I gave up. I had (and still have them) 5 other cars to work on. I just couldn't see spending $40K just in body work, and I did not want to learn the very specialized work needed to work with the aluminum body and to have to buy all the special tools. And it would have taken up a lot of garage space as well as time and patience.
I absolutely love the Double Bubble, Record Monza and all of the other alloy bodied Abarths. If you want one, I agree that you should only buy one in the best condition you can find, and the most complete you can find. And, I would own only this car to work on so that you can focus your total time, energy and money on it. I also agree that you should only restore it to original condition. These cars are too historically important now.
I have been enjoying working on an 850TC replica for several years (at least). I have still wanted a genuine Abarth since I sold my Record Monza, and I just bought a Fiat Abarth 1300/124 to restore. It's not as exotic as a Double Bubble or other alloy bodied Abarth, but it is still an Abarth, with many changes from a stock 850. The main reason I bought one, is that the body is steel and it is very easy to work on compared to aluminum. Plus, many parts are far easier to find, and you can even use a stock 850 body to salvage sheet metal from.
Someone on this forum (in California) recently had a Record Monza for sale. It looked to be in pretty decent shape overall. I think he was asking $40 - $50,000 for it. Not bad when you consider how much money you dump into one that needs a lot of body work and has a lot of missing or damaged parts.
Gil