The late Fernando Castanheira, a/k/a "Hauser" on our forums who passed away last year, had 15" wheels with full wheel covers. I don't think "the look" whatever that may be, was impacted in any negative way by this. The outward appearance was pretty normal and it would take a number of, shall we say, nit-pickers with a red pen and clipboard to really see a difference--and they'd probably have to look at tire size and or another Pagoda with more conventional 14" sizing to discern a difference.
I agree with Cees that it would be challenging to get 500 people to commit to sets of these wheels here on this forum, nearly regardless of price.
I think that doing wheels in appropriate sized 16" may solve the ever increasing problem of finding appropriate tires. However, "be careful what you wish for". People here have been complaining, some vociferously, about the price of tires for the Pagoda. When the Coker Phoenix was one of the only remaining "appropriate" tires nearly 20 years ago, they were "only" $170 each, and few takers here because of the price. Now they are significantly more costly. Even the Vredestein and Blockley tires are too costly for some.
A stock tire for a later model SLK is about $220 when I bought a pair a few years ago for my Aunt's car. I cannot remember if this was a 16" or 17" or the exact profile, but I do remember that it was going to be about $1,000 to get a full set of four. That's still WAY too high for many here, who seek out no-name, or Hankook, or Kumho or some other brand in the $100 range.
My guess is the best answer since this would be a low-volume product, to find a 16" rim with the MB bolt pattern that may fit our car. I don't know if this exists but someone would have to do some research or speak with or go to a knowledgable wheel retailer or distributor and see if any can be fitted. THEN find an appropriate tire not too varying (meaning you probably don't want really low profile tires) that may work. Yeah, it's an investment in time and trouble with no guarantee of success.