I was in the same situation earlier in the year, but I chose the 135i after test driving both cars. The e46 M3 entails higher maintenance which you already know. The 135i does not have the mystique or glamour of the M3, but IMO it is more fun to drive. I do still search for e46 and e92 M3s and plan to buy the e92 in the future. As for now, the 135i is it and modding the car is fun and not too expensive. That said, I did not go with Dinan parts but other aftermarket quality parts
Either car is great but I would recommend test driving both and then wait to see what shows up on the market.