I just bought the rear suspension kits from FCP Euro. That rear suspension is honestly a nightmare to work on, but I got it done without many special tools (just the special BMW lower ball joint tool and a standard bushing press). The air springs are a piece of cake if you plan on keeping the air suspension....I converted to coils which made the job much harder.

I'd suggest removing one piece at a time: e.g., remove one upper control arm, and then loosely attach it to the spindle again before continuing on. For the lower control arm, I jacked up the spindle to move everything out of the way, put the bolt through the two inner bushings, and then fiddled with the height of the spindle and control arm until I was able to insert that giant bolt that goes through the integral link and lower ball joint.

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2006 X5 4.8is