u.s. crutchfield website (mailorder outfit). alpine 6-cd changer is the exact model bmw sells thru dealers. see their fit guide, it should be about $300. fits in the trunk, car comes prewired for it, and i think you can control it thru the unit you have in the dash.
the in-dash thing would be a heinous to replace due to cost. not many new wrecked autos either to get a swap, i think forget it (someone here may know better though).
i always wondered why they do this in e.u. with the cassette as standard, last time i even used a cassette was two decades ago......
p.s. &*(% the cup holder.