ok, the HD is not needed, but is nice to have if you are actually going to be gaming with it...
things you definately need...
1. Motherboard
2. Ring of light main board, with the ROL and power switch on it
3. Disc drive
4. this is iffy, it depends on how good your cooling is going to be. If I took my 360 now, and removed the cooling tunnel and put it back together, I would probably get the ROD because for some reason it gets rid of alot, and I mean alot of head, so if you have a good cooling system and alot of fans for it then you'd probably be good. I would still use the 360 fans because they have a special plug and if removed, would probably cause the 360 to not work, error lights... So you will probably need 360 fan and cooling tunnel