If you're were it's hot, the a/c would be a fantastic addition IMO. I live where it's in the mid 70's, and sometimes in the mid 80's for about 6-7 months a year, and the central California Valley areas are in the 90-low 100's during that time. My car is almost undriveable during the day (on hot days) unless it's just around town with all the windows and the sunroof open. If I take it out on the freeway in those temps, with the windows down and roof open, or windows up and the roof open, it is so noisy, windy, and miserable inside, it's really no fun. I drove it to Lake Tahoe one year, during the week after Memorial Day weekend. It's about a 7hr drive from here. A couple hours into the drive, running up Interstate 5 through the CA Valley, my girlfriend and I were so sweaty and and miserable from the noise, the heat, and the buffeting of the wind through the windows and sunroof, that we were literally regretting taking the car. By the time we arrived in Tahoe we were just wrung out. Love my car, don't get me wrong, but it is no fun in the heat. And unlike the W108's, the Coupe's have very little ventilation into the car. You have a choice of floor air flow, or defroster air flow. That's it. I have no idea what the Germans were thinking when they designed the car. My 1970 Austin America is built the same way, and it has a completely black interior!!
And my 2 cents about the sunroof. It's a beautiful look to the car, and I'm very happy my car has it. It is completely non-functional, except around town. It makes the car so loud out on the road, you might as well be on a motorcycle without helmets. Plus, if you think you're going to open it on a hot day, and rescue yourselves from the heat.....wrong. Depending on where the sun it, one or both of you will just be sitting there cooking in direct sunlight. Miserable
If you haven't already, take a look at the Vintage Air set ups. You may find a combination of small compressor, small condensor (the radiator looking section that will sit in front of your car's radiator) and interior blower unit with evaporator (what makes the cold air) that will work well, and look very period for your Coupe. And it will be sized and set up for R134a refrigerant, so it will make good cold, even at low speeds and idle.