Keep an eye on Alpine. They have things for this sort of thing, a control box for Ipod hookup, and I'm pretty sure they have satellite radio too.
Of course, the other, grossly expensive way to do this is to take the car to the dealer and slip them some cash to influence the install of a newer audio system control wired for satellite, as well as the Ipod hookup gear found in the new 3 series. It would be a custom job, and likely very exepnsive, but I'm sure its not impossible, and there are few places that know the system better than the dealer.
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Changed the car's name, now it's - Roxanne -1985 325e