Bluetooth options for you are two:
1. Get RCD510 (cheapest in your case) etc unit plus bluetooth module with loom.
or
2. Get RNS315 with built in bluetooth.
The main point is that RCD500 doesnt support bluetooth and red fis doesnt work with new bluetooth modules aswell.
There is 1 one more way, and the cheapest, if you have MFSW, you could get 1k0 730D bluetooth module, which works with all phones, supports MFSW, but doesnt show anything anywhere, etc headunit or fis display. Can answer/end call via wheel or phone, calling only from phone, supports bt audio.