And unless Oxford has a significantly higher proportion of white-tie events than Cambridge, I'd doubt you'd need one at all, and would hire for any occasions you did need it. A dinner suit might be more use though.
I'm coming up to my second year and so I like to think of myself as knowledgeable on this subject.
Subfusc is used for matriculation and prelims only.
You will need a dinner suit depending on how many balls you plan on going to, there are loads of college balls in trinity and there are a few in hilary and michaelmas so I'd say its worth just buying it all yourself.
So (for men, evidently), Sub Fusc simply requires a simple dinner jacket and a white bow tie? Must we use the shirts with the funny collars (sorry, from Yorkshire) or will any plain white shirt do?
Sub-fusc for men includes a dark suit (NOT a dinner jacket) and a normal shirt with collar, not one of the things you wear with a dinner jacket with the funny collars. You'd get away with wearing a dinner jacket (probably) to matriculation, but you might feel abit silly. Better to get a normal suit.
Bringing a dinner jacket is a good idea, since there will be the occasionaly black-tie do (the only ones I've been to were eights dinner and our college ball), but alot of events will say 'smart' on the dress code, so a normal suit would be best for them too.
This is the sort of thing you should be thinking along the lines of.
As far as I know, your shirt can have either the wing or the 'normal' collar. Lots of people have the wing collar (as you have to wear a bow tie, and wing collars are meant to be worn with bow ties), but lots have the normal one (as it is not a dinner jacket you're wearing, you're not meant to have a wing collar). I think it's roughly an even split between the two, so get whichever you want.