So many "fors" and so many "againsts".As someone who restored a 1964 Olds 98 convertible several years ago - including an engine rebuild - I can attest to the fact that parts for classic American cars are not always that easy to find, or even very cheap once found.So many American cars were shoddily built in the mid to late 1900's while Mercedes probably was reaching a peak in quality at the same time. One of the examples you gave was the Cadillac Allante. I really liked that car, in fact it was the car I was going to buy 15 years ago when I ended up with my 107. After talking to an experienced GM(Cadillac) technician I learned that all the mechanics shuddered when an Allante came into the shop for service. Apparently they're real pigs to work on.. What it really boils down to is personal taste. Although I admire many older cars of many nationalities it was the 107 I eventually chose and although it is a European model it handles the North American roads with aplomb, purring on the highways and growling on the byways. I have no problem in taking it on longer trips, but maybe that'e because I believe in preventative rather than repairing maintenance.The 450SL is designed to drive at speeds far in excess of anything allowed in America. This means they are a lot more complicated. More complexity in older cars means more parts likely to break down. The cost of MB parts is much higher, and often takes weeks to procure even from the dealer. The cost of repair at MB dealers is way too high, and the indy shops hard to find outside of prime-wealth areas.
As a weekender a 450SL is great. As a tour-America car where reliability is important, and where available parts and knowledgeable shops are everywhere American iron means you are on the road, not stuck on the side of the road with the hood up. Then there are the road conditions. 450SL is made for Europe which either means fantastically smooth and perfect engineered autobahns with no speed limit or 135kph outside of Germany, or ultra-twisty mountain roads begging to be driven fast. American iron is made for American roads which means badly maintained interstate highways with 55-75 mph speed limits, ultra-wide secondary roads where cushy suspensions are more important than cornering or braking.
If it was me, I would buy a 1966 Cadillac convertible with peeling paint that was well maintained by a cranky old guy who drove it to the end and died last month. Actually, that was inspired by a friend of mine - from a famous car family - who was living in NYC and watched some car movie I cannot recall. He and his buddy took a cab to LaGuardia, flew to Phoenix and bought a 66 Cadillac at 2 a.m. from a vendor who could not quite understand what was going on but was happy to take their money. They drove it back across the country with the top down and a case of Veuve Clicquot in the trunk that they slowly drained. A few weeks later he came up to our summer place for the weekend back when all the guests were single and beautiful... fitting six in the Caddy with plenty of room for everyone.