As far as I know you need one of the non-FW 800 boards, and the special MDD OS 9 CD. I hear Apple will send you one if you explain your situation. I also hear they'll send you 10.5 so you can run a modern version of iTunes and buy digital content from them (or something like that). Someone else will have to confirm that.

Since you said you were looking to stock up on parts, you could buy a complete MDD capable of OS 9 and sell the motherboard to help fund other purchases. Everything else should be good for spare parts. Or sell the motherboard and use the funds to buy one that'll take OS 9?