If you decide to go with a 16, remember the guys going fast upwind will usually take the race. And going fast upwind takes a highly tuned boat. But you'll have an easier time economically bringing a tired 16 to standards than an 18. Here's what you'll need to have a good 16:
1) New sails
2) A single bullet block to rivet onto the front of the mast to distribute jib tension evenly for optimum mast bending.
3) A good downhaul system
4) Adapt with new blocks the jib cleat and traveler system
5) New shrouds that permit heavy mast rake
6) Low profile blocks to allow block to block sheet tension for fast upwind performance.
7) A very well tensioned trampoline, and good hull alignment.
Good rudder alignment, rudders don't have to be expensive but well aligned and you might have to redrill them if they cannot be adjusted (older boats) for rake.
9) Of course you will need a fully functional double trapeze system.
As the other fellows have mentined, having a finely tuned boat for competitive racing is expensive and there is no easy out. Believe me, I had to tune my 1979 H16 to new standards in 1993 and it was not cheap. I'm not sure if I had been better off back then buying a new one. I eventually did.