Several times in the past few years I have pulled about 4,000# of trailer and load on 1200 mile trips without issue.
IMHO, the keys to reliability are:
Don't buy a trashed out, beat on vehicle with mega miles on it that has been neglected in parts maintenance/replacement. Stuff wears out! I bought my X5 at about 78K miles and I believe that is the sweet spot of mileage to buy a used vehicle of almost any brand. If you're buying a 200K mile X5 that has not had good care, you're just asking for reliability issues right off the bat.
Replace components BEFORE they leave you stranded (bottom end of engine and other major engine parts notwithstanding). How much does a 110K mile ball joint owe you? How much does a 110K mile fuel pump owe you? How much do 110K mile heater solenoid valves owe you?
If I'm replacing something and I have to touch/remove other high mileage things to get at the item I am replacing, I simply replace the other aging pieces.
I don't necessarily buy all or many OE parts, and that is my personal choice on how I manage my maintenance costs and I have been satisfied with my choices.
Yes, achieving high reliability of older high mileage vehicles costs money, but in my view, far less than the $30K or so that a newer vehicle would cost me.
Mike