Like DBD mentioned, you must try them on before buying. Would you trust buying any shoe without trying it?
Some mtb shoes have stiffer soles than others and that can make it a little awkward as the shoe might not bend too easily under the ball of the foot. Effect is that you might slap around as you walk as if you're flat-footed. Shoes are a personal thing anyway. I like Shimano shoes that have the velcro tape which is handy for keeping the lace ends out of the way. You don't want them getting caught on your chainring, eh?
I had a pair of very thin road shoes but I did not feel happy with them off-road as I felt a bit safer having something sturdier around my foot that gave better protection. A pair of trainers will do nicely as well.