19's may be a bit big, because the larger the wheel, you will have to get proportionally smaller tires, i.e. worse ride quality. I would suggest 18's. I myself have 17's on my '95 325i, and i am completely happy.
As far As suspension upgrades go, you do not need to upgrade suspension to accommodate new wheels, if you are maintaining the same overall rotating diameter. (this means the new wheels and tires overall are the same diameter as the stock wheels and tires- this is a good idea, as it maintains accuracy of your speedometer)
I did not upgrade the suspension on my BMW right away, but because i wanted to lower my car to make the fender wells fit around the tires better (less gap), i got H&R springs and rear bilstien shocks.