I replaced the steering box in my 500 SL a couple years ago with a rebuilt one from
http://www.************************ . Paid about $250.00 for it during one of their 15% off sales. Their current catalog lists it for $465.00. You may be able to call them and finagle a discount over the phone. FWIW, my mechanic said it was the best rebuilt one he'd ever seen.
I've probably put 20,000 miles on the car since the replacement and the steering is as tight as the day the box was replaced.
Some folks here badmouth PP, but I've had nothing but good experiences with them...even with a couple of item returns.