As others have said 4/5″ is perfect though people do it on all sorts. To be honest unless you are intending to win you’ll have fun on any bike (within reason, Tesco special excluded). The track is not difficult, just long and therefore knackering.
There’s no camping in ADH other than next to the lift in a dusty field with no facilities. If you’re doing down that route I’d camp at the site down in the bottom of the valley at a place called Allemont. The bloke that runs it is a friendly fella, it’s a nice spot, not expensive and there’s a pool along the road. It’s also well served by busses with bike trailers so you’re not cut off. The race ends there as well.
It’s a great event and I can’t recommend it enough. I was there for the first time last year and I’ll be back!