Other than going with OEM dampers I'm affraid that Billies are your only option near the OEM price point. A good coil over set is is going to cost >2K and if it didn't then I would not consider it to be a quality product. Short of AST Suspension making dampers for the E53 I think your only options are KW, Billies or OEM.

Personally I would try to drive, or at least ride in a E53 with Bilstein HD before you look them over. Personal perference can vary widely and you might actually like the ride of "X" damper over the opinons of others.

FYI - If you want to price Bilstein HD out I would start here: bilsteins anyone? - Bimmerforums - The Ultimate BMW Forum

I've ordered from this vendor, for another vehicle, and he was able to beat any price on the web.

k