Noob looking at stereo upgrade options

[Sablotny]

Forgive me if I post this query instead of searching too hard... just bought a 2001 3.0i with nav/non-DSP. Looking to upgrade the stereo and wanted to know what's easy/popular/been done. I like the Stage 1 speaker kit from BSW- I'll probably go for something like that.

Lastly- is there a single CD player available to replace the cassette player behind the nav unit? (Seems like BMW and Acura NS-X drivers were the only people still playing cassette tapes into the 21st century). Even using this input for an XM radio would work.

Thanks for any help. advice or experience. Not looking to drop $10K on my system, just get better sound while keeping the stock look.