A couple of remarks from my side (but better wait for others to confirm/deny/add) before concluding anything.
I see the price of $74k, which is, I think, is quite acceptable for a nice car with minor imperfections.
- decreasing idle rpm on switching on of air condition as well as air condition blowing warm air may turn out to be easy fixes (let's hope for that)
- radio, seat covers (I see lots of wrinkles e.g. on the side of the driver's seat - something has to be fixed there), seat belt, steering wheel, couple of minor imperfections in the engine bay - all these details to me is fun rather than trouble
- turn signal switch should cancel; can be broken, may require just some small fixes, may be wrongly installed or may need to be replaced (this is some dollars to spend)
- documents look great
- underside looks as well taken care of but a driven car - which is fine
- the car looks complete, I think, nothing major missing
- I am sure you decoded options from the options plate to see if they are all there in the car, just as you decoded the original colour
- I would not worry about replacement engine, I would not worry about mileage (which always can be a mystery if the car has gone through restoration)
What you also see:
- the seat covers were replaced
- the carpets - I see heelrest pads as I saw on original carpets, I also see quite "cheesy" finish of the gear shifter surround, looking like finish of carpets of cars from former communist block of the time

, but I think we came to conclusion on this forum that this is how it looked for late 280SL production - so carpets may be original
- you car was built for American market - and has Euro lights
You may want to not get distracted by details such as tires. It is a tiny bit in the whole equasion. Just do not drive on old tires!
You will be looking at more critical things, I am sure:
- are the gaps between body panels even?
- if the body bears traces of repair - has this been done in a professional way?
- is the paintwork on aluminum body panels ok (bonnet, trunk, soft top bay, doors)
- do the four digit numbers on body parts match the one from option codes plate (tops, bonnet, gearbox support)
- is everything working - blowers, wipers, etc. - repairs are somewhat difficult there
- rust - is it really rust free - and you have lots of good tips on this forum on this (check those on upper parts of fenders, from Dr Benz, I think); personally I do not believe it is rust free, but I hope I am wrong. Also: some rust can you can get rid of at a price that will not make this deal flop.
- how the spark plugs look like, how the tips of exhaust look like, etc.
- does the engine blow any smoke on revving up, or just all the time
- are there noises that you would not expect to hear
As for the drive: these cars handle really well. You should not experience any odd behaviours while driving - if anything like that happens - something is wrong. And they are not the most quiet of all cars. But it is a regular noise, strange noises you would detect, I am sure.
Look at this:
https://www.sl113.org/wiki/Buying/PrepurchaseEvaluation This is the "Buyong a Car" section from Manual on this forum. A must see one, but I think you went through it already.
If the car is somewhat rust free - I think it is a good offer, as long as there are no dead corpses in the closet. I think it represents an offer worth pursuing. You have in the US a lot of specialists from this forum to provide advise on your purchase - they know every nut and bolt in these cars. You follow their advise and you can sleep tight, just have fun fixing the minor imperfections yourself. And it is fun, believe me, like Cees says as well.