MUN is located in the centre of town. If you get an apartment in that area getting around won't be an issue (bus or walk, bike when weather is ok). I'd say when you get into MUN you will potentially find roommates.

Social life won't be an issue for you if you are a little outgoing. There will be a ton of socials and stuff with MUN, and there is a very active night life in downtown St. John's (look up George St.). Generally you will be around a bunch of people in similar situation to you.

Lack of diversity? It's not an overly diverse city but there are lots of different restaurants around. Overall Newfoundlanders (Pronounced New-fin-landers, say it right for bonus points!) are very accepting of anyone who isn't an asshole.