Don't have a comment on value, but only consider restoration if it's for yourself, not for resale as it will cost more than it will be worth (probably ever given the rebuild title).
Unlike American cars, "matching" numbers don't really happen with MB's so you can feel free to swap in a correct or updated engine from another model, same for transmission.
Most all MB's need meticulous attention to maintenance, I have spent probably $6,000 (minimum, maybe $8, I try not to think about it) on mechanical work for a car that was driven regularly when I bought it so yes, you will have a project ahead of you if this was left outside. You have to consider that brakes are probably frozen, all seals need redone (they are surprisingly costly for Mercedes and you will need to use OEM, especially on a roadster, or risk buying cheaper ones only to have to pull them out and buy the OEM once they leak anyway). Interior parts are unique to the car and vary by period, not cheap. Tank is likely full of rust or holes if the floor is shot. Engine could be seized? Then again, many old Benzes have surprised lots of people so see if the engine can be turned over by hand, do a thorough check/cleaning of fuel system before even attempting to start the car, as with any car that has sat, put in fresh filters and fuel, or feed from a secondary source, new plugs, etc., and see if she starts.
Maybe you can leave the floors out and have a double-sided convertible.
Pics are always useful, of as many angles, interior, engine bay, etc., as possible.
Oh, and the theory of several hundred test models is nonsense, Mercedes-Benz only released a car when it was fully engineered and ready for debut, at least back then. Maybe her data card shows it was built or ordered in '72 but was delivered and sold as a '73 model which is common among many manufacturers.
Finally, check under the R107 section under "Roadsters" subforum, that is the car you have.