Mr.B, there a MILLIONS of motorcycle riders on the highways (and off road) that don't let the lack of a spare tire destroy their enjoyment. So don't let it stop you from enjoying the best sports car bargain on the planet.
As Inferno Red pointed out, some people have purchased a Mercedes donut for extra peace of mind, and having a good plug repair kit is a great idea.
Different tire diameters shouldn't cause any significant problems, some Mercedes models come with different front and rear tires and still come with just one spare tire.
AAA isn't a bad idea either.