Stay away from the 5.0 L. The V-8 is noticibly more in maintenance costs expecially if you start to get a bunch of miles on it. If you wan to keep the vehicle the valve timing chain guides are a pain to replace, which means more time to do the job and cost more money. The straight 6's require less expensive maintenance and if maintained should not need chain guides for a long time. 15K miles per year are an average ask for maintenance records if there are any. You should research standard maintenance costs for brakes, transmissions, average engine anti-knock sensors, ETC. The X series vehicles seam to require more than usual maintenance than other BMW's or other SUV's.