If you have no clue on what boat you want to buy, then you had better do some test sailing to compare. How easy is it to move, set-up, sail. Is it going to be beyond you skill level or something you will quickly get bored with?

If you have narrowed the search to one of the Formula class boats then again go for a test sail on the various brands. There are subtly differences in set-up and feel between brands that if your skill level is sufficient you may notice.

If you know what kind and brand boat you want then a test sail is not so necessary. Make sure you (or get someone who knows the boat) carefully inspects it if it is used. All the parts need to be there and the sails, rigging and hulls need to be in a condition that is representative of what the asking price is on the boat.