I would hazard a guess that you stand a reasonable chance of correcting this problem for a lot less money. Sign up for several of the BMW enthusiast forums and search their archives for key words and your question (although I have found the Car Talk forum to be as good as any forum on the web)
Consider junk yard parts, and also look for shops that specialize in rebuilding the hydraulic control units. We had an ABS unit from a Volvo S70 rebuilt recently for a small fraction of the cost of a new one. Works just fine.
If you strike out all around, it is gut check time. ASC is really nice, but BMWs without ASC are still safe, great-handling cars. I have a '97 with the ASC out right now and I am going through the same process you are - but I promise I will not be investing $4k in that car. If I have to, I will just pull the warning light out of the dash.