I did this last year, really well run and a great atmosphere. I did find the course very hard going on your tyres in certain places (as in multiple pinch flats on multiple competitors), so I'd strongly recommend dual ply tyres or going tubeless. As for riding a hard tail, well if you've never ridden anything else then go for it, if the option of full suspension is there then I'd go for that - but that's just down to personnel choice.
The terrain was a combination of grass, moorland track, gravel and rocks (lots of rocks, hence the name!). I'd definitely recommend armour of some sort, I remember riding quite conservatively and that was with knee and shin combo (normally do anyway), elbow pads and a normal helmet. There just always seemed to be some jagged rocks of some shape, size or description there to soften your fall!!
I found the trails different to most stuff I'd ridden before, so it's a good way of experiencing riding something new.
As for the entries, e-mail the organisers, they're good with replying to questions.