when college send you your welcome pack, they should tell you how to sort out gowns. if for some reason you don't get one in time you could always ask your college parent if you could borrow theirs.
i didn't buy any books in my first year because the college and department libraries had all the books i needed. you shouldn't need to buy any books before you come. it's best to wait until you get here and have settled into the course, with advice from your supervisors/director of studies, for a few weeks before purchasing anything.
we were told for matric photo - suits or smart trousers/shirt/tie for boys and black trousers/skirt and a blouse (traditionally white but it's not compulsory) for girls...plus the gown obviously.
for formals, girls would wear smart trousers/skirt plus a smart pretty top/blouse and boys suits. you may want to bring a cocktail/prom style dress incase you have a black tie event such as boat club dinner at the end of term. for boys, it is probably worth investing in a dinner suit because you will get events to wear them to over your 3/4 years making buying one work out cheaper than renting each time.