I would. An E9x M3 is such a special car IMHO. If you can afford it (plus extras like more expensive tires, oil changes, ect...) and the big decrease in mileage, then go for it!
While the parts aren't exactly cheap, I personally think the M3 is even more reliable than a 135i to be honest.
Current: 2008 BMW M3 Sparkling Graphite/Bamboo Beige/6MT. 1999 BMW M3 Techno Violet/Dove/5MT. 2007 BMW Z4MR Alpine White/Imola Red/6MT