Recommendations on Transmission Rebuild?

[3050rpm]

A buddy of mine has a '94 540i with an automatic transmission on its way out, in particular noticeable slippage in first gear, the others seem to be okay. In an effort to save it, he's had the transmission opened up to drain out and replace the fluid (although this is supposedly a lifetime sealed kind of thing) but not to much avail. He's been given a price of $10,000 for a new (presumably rebuilt) unit, or $3000-$4000 to have his rebuilt. The car is otherwise fine, even escaping that notorious problem of the sulfur-content fuel coming in contact with the cylinder walls.

I seem to recall some apparently well regarded place I came across on the Internet that does BMW engines and transmissions, but I can't recall the actual name. I would appreciate any advice, names, recommendations people may have, it seems a pity to have to part with an otherwise good car. Thanks.