Good luck finding a BBA
back in the day when they were still available from Sega.com I had an opportunity to buy a few for $50 a piece I didn't really need one at the time and like an idiot I decided to to wait for the price to come down...
no less then a month later you couldn't find one and if you did you were paying $100+
at this point you'd be lucky if you can find one for less then $200
I used to have some PC application that would allow you to connect the DC modem to your PC and connect to the servers as if you were using a BBA... I'll have to see if I still have the disc floating around.
I recently picked up a copy of Virtual On Netlink Edition to complete my Saturn Netlink Collection (yes I even have Daytona Netlink... though it's in rough shape... no none of them are for sale). Netlink adapters are fairly cheap but most of the games are skyrocketing in value.
I also used to have a set of 4 Agetec Dreamcast joysticks... I bought them for $10 a piece whenever I saw them on discount racks at EB or Gamestop, only ever used one of them and resold them all on eBay for about $150 a piece a few years later
As for coders cable, I have one somewhere, I used to rip my own DC games with it (painfully slow). I even have a cable to connect VMUs directly to your PC through the serial port.... neither are for sale, you'd be better off just building your own, they're not all that difficult to do.
What I'd love to do is get a hold of a BBA and reverse engineer it. you could make a killing dropping them into modem cases and hocking them on eBay