Don’t go for a full on DH bike, it’ll slow you down more than it will speed you up, seriously.
I did it on a Cannondale Gemini DH last year, not the longest travel or heaviest DH bike in the world by a long stretch, but the slackness meant for the bits you have to climb it was a bastard!
My mate did it on an 06 Spesh Enduro funnily enough, and though his OEM Marzocchi forks were a bit pants for it, the frame was plenty ok. If I was choosing a bike purely for the purpose of it being a good bike for the Mega, I would say it would probably be one of those Lapierre Spicy’s. Long, stable, slack, very efficient suspension both ends, light enough to pedal uphills (this is REALLY important, I can’t stress it enough, though use a chainguide and a 34 or 36T ring with an 11-32 or 11-34 cassette though). The only 2 things that need to be "DH" really are the tyres (Maxxis dual ply really, or Michelin DH, preferably run tubeless) as lightweight tyres won’t get you far without punctures, and the brakes (you’re descending for over an hour non stop, you NEED powerful and reliable brakes!).
The rest of the kit, well people can and do ride their XC bikes to do it. It’s just how seriously you’re going to take it though really. Oh, and don’t think you can just rock up and do well. I knew I wouldn’t be fast, but I didn’t even qualify for the race! I rode well in the qualifier I thought, there are LOTS of very good riders that do it.