I'm not sure I can be that helpful, but I'll tell you what I have just learnt.
I finished my first ultra yesterday - Keswick Mountain Festival 50k. Much of the route was steep and rocky. I ran in Mizuno Wave Ascent, and they weren't up to the job at all. They're basically a road shoe with a bit more tread. I looked on enviously at my fellow runners with Salomon and Inov8 on their feet.
I'd say it depends on the terrain. If you're running on trails and paths, then you don't need anything special. If it's going to be rougher under foot, then a fell running shoe seems to be the thing. My Mizunos were great for the last 3 kms on the road...but pretty rubbish for the previous 47km...
Good luck with your search. I'll be interested to hear what those more experienced than me say, especially about the wide toe box as I have wide feet too!