The first thing that comes to mind is.... "What do you want to do with it?"
Car shows, tours, short road trips, long road trips? Do you want to just get in and drive off or are you okay with finding a car that has some minor needs (they ALL have, at the very least, minor needs).
The money, not what they cost, but constantly thinking of them as 'investments' can get in the way of the pure enjoyment of one of these old cars.
It always makes sense to buy the nicest car you can find in your price range... correctness counts too, but do you pass on a car with an aftermarket radio and holes cut for speakers in the back just because it isn't as 'correct' as it could be? I guess it depends on price and these other factors.
I bought a great little 230SL, Euro spec w/ Euro lights, manual trans, kinder seat, about 2 years ago for $20,000, a steal, even then. It had been repainted in its original color, the interior was original MB tex and kind of worn, the hub caps were from a later 280SL (one piece, incorrect for a 230) and weren't painted to match the body, they were just chrome, it had an aftermarket radio, the soft top had a hole that had been patched... but it was $20k and I don't think anyone with a car they paid 3 times as much for had anymore fun than I did in that car... driving it thousands of miles in the one Summer I owned it. I talk to a lot of guys who haven't put 3,000 miles on their cars in 10 years, sad.
I was hooked after the summer of 2014 and I decided I wanted a nicer, longer term Pagoda... so I kept my eyes open and one came along. A 250SL with an interesting history.. so I sold the 230 (to a guy in Belgium who was happy to get a Euro car, it's undergoing a complete restoration just outside the city of Ghent)... and bought the 250SL.. that is, more correct, original and all of those things that make sense for a car I plan to keep for a long time.
If I hadn't had the wonderful experience with the driver quality 230SL, I probably would have just sold it off and gone back to an air cooled 911 or something, but the Pagoda experience is one of class and sophistication... no matter what that middle digit on the back is...a 3, 5 or 8.
I think the 230s represent a real value at the moment, they are still available for far less than a 280 and, in my opinion, the way we use these cars today, as pure, leisure vehicles, just as enjoyable and usable. They made many more manual 230s than 250s or 280s... so they're easier to find.
I believe that sometimes the right car just presents itself to you... and maybe it will check all the boxes you've imagined, or maybe it won't. Maybe it's a U.S. spec car with Euro lights and no kinder seat, or some variation. My 250 is an automatic and I NEVER thought I'd buy an automatic, but everything else about it was too good... so I went for it and am very happy with my decision.
There are plenty of cars on the private market. Resources like this forum, your local MB club, are both good resources... event the local, independent MB shops in your area.. ask them if they know of any of their customers that might be interested in selling a Pagoda SL.