I have wide feet and can't wear the kanadias or many shoes for that matter. You really need to go try a few pairs on.
As has been said, depends on the type of trail you are running on. I run a trail race in Aug and would not want to run it in road shoes as the ground can still be damp, wet, muddy, the slopes steep and trippy and the streams still wet.
Maybe if you ask on the events page for anyone who has done the race before.
I'm running a trail race next week end and I will probably wear my new shoos - inov8 268, I've done 3 runs in them over the same terrain as the race, as it's where I do my long hilly runs, though my runs are far more challenging.
Also, I think that it would do you good to train on similar terrain, maybe in your road shoes, or whatever you choose to buy.
Once you've run off road you'll wonder why you ever ran on the road. 
Look for something in the sales / last years model. You should be able to pick up a decent pair of trail shoes for £50 or less. I've been wearing New Balance 101's, I think I paid sub £40 for the last pair.
Edited: 13/06/2012 at 19:43