If you can't afford to rent an outfit (jacket only), I like the black vest idea. You can buy a cheap one--even a used one at a consignment shop--for that matter and sew on some silver buttons on the cheap. Then add a white shirt and black tie and boom! You're pretty dressed up.

The only problem investing in a really nice (read: expensive) outfit at your age is that you are likely to gain weight over the next few years. Even us old guys outgrow them.

Good luck. YOu'll look great.