depends... if you would have to sell a child for new tires (800-1100$ a set) -then no. bmw wont give you a warranty after 60-70k miles but if you have one before 50 and you update it with them you would be okay. 3rd party warranty is an option too. like the guy above said , there are
a bunch of posts to look over about your question.
if you pick one up thats been beat to hell or say a red salavge

you will have problems. a well maintained X should work out good for you.
there are guys on this site with a million miles on their cars with not that many problems at all.. do your homework and avoid the molested x5s with fresh wax...
oh by the way , the 4.6is is cooler dont listen to the 4.8is guys ! find a 4.6is and the ladies will like you. it's a guaranteed fact of science.
when they see the 4.8is they say "but the rims arent as cool" and "the front is messed up"

(honestly 98.9% will have no clue )
gl with finding one.... the 4.8is\4.6is are near perfect cars in my book