X5 E53 4.8is Reliability

[Mr.Wrong]

Hi fellas,
Have been browsing this forum for a while and now just signed up since I'm stuck with a decision and need your input on this one.
I've been huge fan of E53 generation for quite sometime, especially after the facelift and the 4.8is addition.
Now I've wanted to purchase 2006 4.8is for a while, but when faced with purchasing a new to me vehicle in 2009, I went with 2005 S4 instead of 2006 4.8is, purely because it was out of my pricerange at that point. Now I can finally afford my dream SUV 2006 4.8is! However, I'm stressed whether it's going to be worth selling Audi for X5. My main concern is reliability! It's kind of dealbreaker.
-So how realiable is the 2006 4.8is?
-What should I watch out for? I know one of the common things is Air Suspension, which I believe could easily be fixed with a set of CoilOvers, unless I'm wrong?
-How long should I expect it to serve me before it becomes unbareable money pit(100k, 150k on the clock) I need the car for about next 5-6 years without giving me a lot of trouble besides regular maintenance. What's your estimated annual upkeep dollar figure on the car(excluding mods)?

The reason why I'm thinking of selling S4 is mainly 2:
1. Should've gotten it in manual
2. Need a larger size car, S8 is out of my pricerange at this point so X5 is first in line.
Other than that Audi have treated me extremely well, and wouldn't even think about selling it if it were at least 6-mt... So keep S4 or switch over to the darkside

Thanks