Welcome to the neighborhood alps! As for your radio dilemma, here are a few suggestions:
1 - For about $100, you can add bluetooth streaming and calls to your stock head unit using iSimple TranzIT. I worked well for me.
https://www.amazon.com/iSimple-Hands-Fr ... 339&sr=8-4
2 - They have another version (iSimple IS31) that doesn't do as much, but might be worth a shot as it's less than $30
3 - If you want to upgrade your head unit to something rather period correct, Nakamichi made some really good head units in the 90's that I think look solid in an e28. The Nakamichi CD-500 is a really good unit, if you can find one, but clearly was made before the time of bluetooth. That being said, it works well with that iSimple TranzIT solution, so if you do find one, you can always add bluetooth that way.
Anyhow, if you figure out a workable bluetooth solution, definitely post it here so we know what you came up with!