Aside from head gaskets blowing, the 525i is a very solid engine. Not sure where yu heard that it can have quite a few problems, but they where mis informed.
As for the trans, you best bet would to get a reman or good used low mile trans for it. rebuilding an automatic is qutie a bit different than a manual. $900.00 seems high for a car that can't be driven, since it is essentialy a parts car if you can not test drive it.