Before you bother with a flash you need to check with the venue. A P&S flash from the stands is one thing. A good flash unit up close to the action is another. You might want to check with the venue first before you drag a flash unit along unless you want to use it pre and or post fight.
The focal length you need is going to depend on where you will be located. When I am shooting I am always, ringside, court side, sideline. If you are talking ringside then a 50mm or 85mm would be good for a 20 foot to 30 foot ring as would a 35mm. Ring size is going to depend on which promotion is putting on the event. UFC is usually a 30' octagon whereas Pride is a 20' ring.
If you are unsure of the distance you will be at then you will want something in the zoom range, say a 70-200 f2.8 or perhaps a 24-70. Not knowing the venue is always a problem. What ever lens or lenses you choose make sure they have fast AF capabilities. While one would think that the 85mm f1.2 would be idea, it is slow to focus. That's why my sports 85 is the f1.8. Fast, sharp and quick focus.
If there is any way for you to get to the venue before hand and see what you will be working with it would be worth the effort. Good luck.