Based on your age, size, and street experience, I would think that a 250 smoker would be fine, as you already understand how dangerous stupidity can be. I do not agree with you assumption that "riding on the street clearly shows how important the proper gear is!", since I never really took safety gear seriously until I took to dirt and put myself "in harms way" on a bike. All who have seen the cruiser riders in the shell helmets know what I mean, of course, I left my little helmet in Georgia when I moved and only brought the full face for street riding. I digress, and am not sure that your brother really knows what he is talking about. While the smokers do prefer to be opened up, you can run a hotter plug (brain drain occurring, but I know it is numerically lower) and modify the jetting for good riding at lower speeds while learning. Two strokes are also known for their sudden, violent, power hit, but with a larger bore, even that isn't really an issue. As for the gearing, yes, it will probably seem more mellow, even weaker, but you shoud probably try riding it a few times before changing the gearing, most likely you will be changing it back in the very near future anyway.
For reference, I am over 30, and when I started riding I was around 200 lbs, my first and only bike so far is a KX250. I also have a few thousand miles of experience riding cruisers on the street.