First, both the BM2400 and the BC9000 have built in RIAA on the PHONO in... so one hurdle out of the way.
The RCA -to-> 5 PIN DIN is indeed what you need, but $50 sounds awful strong.
Polks are fine to connect, you don't need an adapter per se, but will need a pair of male 2 PIN DIN connectors.
Both items are regularly available on eBay or similar. Heck, I am sure I have both items sitting around???
They are both excellent, iconic pieces. The BM2400 is a classic (one of my favorites) and built like a tank. Most are due for either bulbs and/or caps now. The BC does offer the advantage of the built in CD player as well as TAPE. Problem is that the CD player is a known issue when the clamping mech fails. The BC doesn't ship as well, either. Watch out for sprung doors, cracks in the glass panels. The BC also has a much bigger footprint if location size is an issue.