E39er buying a X5 - which one? need your opinions

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Hi Everyone, Yes I am new here (Bimmerforums) and am looking to spend a little more time here because I am about to buy an X5.

My knowledge, cars has always been with 3ers, 5ers - and my current daily driver - 99 ACS 540i needs to part and replaced with a X5 (damn kids :-) )

The X5 I want is the E53 4.8is - Im in love with it....but wanted you guys to answer some questions on the SAV:

* Off Road - define off road - are they capable to go into Jeep territory, or just simple terrain - dirt, grass, mud etc?

* Which is best - 4.4 or 4.6 or 4.8? (E53 only, not intersted in E70). I want comfort, pickup/torque, options, reliability ( I know the M62TU was replaced in 2004 with a new 4.4 V8 which is best?)

* Best MPG (not interested in 3.0 / 3.0d)

* Quiet cabin

Appreciate your feedback, Im looking forward to buying one of these!!