I suppose the 500 has a smaller overall glass area than larger cars, so the solar heat effect will be less. But I spent a long time with my 500 with non working a/c and found it could be unpleasantly hot on the rare sunny UK days in mid summer.
With the fan full on, and windows open where possible (ie not on the motorway at speed) the temperature was bearable in the same way as a room fan moving warm air around is a bit more bearable than still air.
Once I fixed my a/c, I was delighted to once again be able to source air with a degree of chill when temperatures rose. And yes, keeping the windows demisted in wet weather is an added bonus.
Having said that, I always used to have cars without a/c (never used to be an option) and if Bella in Oz can survive without it, I guess so can you.
With our climate, there won't be an awful lot of days when you'll be muttering "Wish I had a/c" So, enjoy your car!