If you haven’t already done so I would suggest you read a few of the posts in the main E53 forum on this site regarding fuel economy for the 4.4 V8.
Just remember people in the US will be quoting mpg using US gallons.
A US gallon is 3.785 litres while a UK gallon is 4.546 litres.
Also, be aware the onboard fuel economy on the trip computer is in some cases extremely optimistic, I would be very careful with people supplying economy information based on the trip computer alone.

While I own a diesel X5 myself I have a couple of friends with X5’s, one with a 4.4 and the other with a 3.0 petrol.
I know the 4.4 cruising on the highway at 110kmph without towing averages around 14-15 litres/100km when calculated even though the trip computer will read in the low 12’s.

If your current car gets 450km for 75 litres that’s 16.6 L/100km which isn’t significantly different to an unladen 4.4 X5.
Add a 2.1 tonne trailer and you will be looking at economy around 25 L/100. This is approx double what the diesel version gets towing a similar weight.

Since you will be towing a pretty sizable load I would also suggest you do some research into the engine torque figures between the 4.4 V8 and the 3.0d, you will find the diesel has quite a bit more torque than the V8. When lugging around a heavy weight it’s torque that counts not power.
I admit that there’s something special about a nice sounding V8, in fact I own one myself (not an X5), but when it comes to towing, modern diesels win hands down.

Also, the body shape from the pre 2004 vehicles is exactly the same as the later E53’s but there were a number of cosmetic changes.
The easiest way to tell externally is by looking at the indicator lenses (front & rear), the earlier ones are yellow while the later ones are clear, assuming they haven’t been changed which is a common upgrade.

Lastly, if your budget will stretch, I would suggest you look at 2004 onward models purely because a number of niggly issues were fixed. In particular the later models scored the ZF auto instead of the earlier GM unit. While they both have issues the occurrence of issues with the ZF appears to be far fewer.