you won't find standard shocks for the 4.8 because it was not available without air suspension. So you'll have to order the 4.4 Bilsteins. as for springs, you can use the factory springs from the 4.4 or go with an aftermarket setup. I used the OEM shocks/springs from a 4.4 with sport package, got everything from a junk yard for $400. I wanted to see how it all worked before investing money in Bilsteins. With the 4.4 springs, the X5 sits at about the middle position with the air suspension. I believe all the after market springs will be shorter. so its up to you on personal preference. when i converted my X5, i couldn't find any info on height, so by starting with the cheap parts from the junk yard, it gave me a starting point.
The X5 rides exactly the same in my opinion with the factory springs/shocks compared to the air suspension. I'm sure with the Bilsteins it will ride awesome. My brother has the X5 now, and he's ready for Bilsteins!
Hope that helps!
2005 X5 4.8is