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These motors are very well built and with proper maintenance can very well hit 300,000 miles. I have seen an E36 325i with 600,000+ original miles on it.
As for the transmissions, BMW started stating in the early 90's that the transmission fluid was a "Lifetime Fluid" and never needed changing. Well, all fluids break down over time. Some of the BMW transmissions are actually made by GM. I have never heard of GM stating to never change the fluid in their transmissions. The way i see it is this, BMW makes more money from replacing your transmission when it dies every 150,000 miles then it does replacing the fluid every 50,000.
Their are many, many discussions on changing the fluid and many opinions from various people. A lot of people recommend it before the milage gets too high. Other people will say not to do it citing the transmission will fail if you do. I have done a lot of research into this, and while it is possible your transmission may possibly fail shortly after, its not directly caused from the new fluid. People will say not to change it do to impurities or shavings of metal that have gotten into the fluid and have worn down various parts like the clutch packs. The claim is that the added particales are what keeps the friction between such items as the clutch packs, and without that they start to slip. While that is possible, it is more from neglecting to change the fluid before increased wear.
If your mileage is still low, i recommend doing it. The fluid helps cool the transmission and when the fluid breaks down it looses its ability to cool the transmission, much the same way coolant or oil does. Heat is the #1 killer of transmissions. Too much, caused by friction or what ever, will cause your transmission to fail.