Equipment improved has improved a lot in recent years, especially sails, so don’t go too old. A 20 year old board might be a bargain, but realistically it is NOT going to have the performance of any modern equipment. Also, the older the equipment is the less advanced the technology will have been that made it, therefore the more it will age too.
A piece of equipment a couple of years old will have great performance potential, but age isn’t just a number. A sail that is 2 years old but has been used in the UK 5 times, compared to the same age sail that has been used every day in the blazing sun is going to be ‘older’.
If you want performance from your equipment the more modern the better (think a couple of years old, ideally), then it’s a case of researching the product and checking it over thoroughly before parting with your cash.