BB is the safest bet from the options you listed. I'd stick with that if it easy to find a something decent fit with your body type. If you have a sort of hard to find size or have hard to fit proportions (like me), you may want to find a local place that could do a M2M suit for you. Around here (chicago) there are at least two places that will make a M2M suit for you for about $500 (cheaper online if you want to chance it - I wouldn't, but everybody is different). Those less expensive M2M suits may or may not be as well constructed as a BB outlet suit, but in my mind a good fit trumps everything.
Also, Navy and Grey are probably the most versatile choices long term, so I agree with your choice. I'd just keep in mind that sometimes Navy can highlight youth, not to say that Navy makes you look young (if you don't), but if you happen to be younger looking, people may notice it more when you are wearing Navy. I don't know if that is a pro or con in the internship you are going to do (hey its sometimes nice to be the young guy). Its subtle, but I definately believe it now that I'm aware of it: http://www.realmenrealstyle.com/mad-...pete-campbell/