That part from Ricks is available from Parts Unlimited too. Same price. Read the article and agree, nothing not discussed here.
Not sure where the $900 comes from. There may be some parts you need besides the two mentioned. Both the gasket for the cover and the generator comes to just under $800. The warranty from BMW is 2 years. Not sure about the Ricks warranty. Probably at least 1 year. I would bet that the stator will be available soon from BMW as a single part.