Before I ask let me preface my questions with "yes I did search and read through several threads offering qualities of both motors."

I currently own a 745i, MB E350, and Ford F-150. My fiancé drives the MB daily and I drive the truck or BMW depending on weather. We are looking for an SUV (X5) for trips to the mountains in the winter and beach during the summer. We have two large mutts who travel with us.

I'm not concerned with fuel economy because I can use the extra write off. I am leaning towards the 4.4 over the I6 because of the extra ponies. My only concern is longevity and reliability. We are planning to keep this truck and like all other vehicles we own properly maintain them.

Are transmission issues in 2002-04 4.4 models a concern?
Which will be more reliable long term?
I am aware on 4.4 models w/ sport package I will have to eventually change to coil springs, but are there any other issues with the sport package I am missing?
I am not opposed to a 3.0 if over the course of 300k miles the motor will be that much more reliable.
Finally is a 2002 4.4 with under 50k documented miles a good candidate at under $18,000?

Looking to keep the purchase under $20k and pay cash.