Booties/drysuit socks keep your feet dry and let you wear socks underneath (I usually wear a pair of moisture-wicking ski socks). Unless the water is really cold, you won't need more than that, assuming you are wearing neoprene sailing boots and not water socks or something. Once it gets really chilly, you can put a pair of thicker socks on, but you might have trouble with your boots fitting.
Without socks underneath, latex drysuit socks can be really hard to put on and off - takes a good amount of talcum powder to keep your feet from sticking
My drysuit, an OS Systems Spirit (www.ossystems.com) has fabric socks instead, which I really like.
When buying a used drysuit, watch out for old poorly maintained latex seals - even if they weren't used, if they dried out or got too much sun, they can rot, and replacement might cost you more than buying a better suit will. Also, I'd be worried about the hood - do you have to wear it to have the suit seal? It's hard to hear through a hood, which doesn't matter if you are wearing it for diving but is of greater importance when sailing. Also, if you can stretch your budget, get a breathable suit - this will extend the range of temperatures that you can use your suit greatly, and keeps you from feeling like you have been in a steamy swamp on any day when getting out of the suit
Just my opinion, of course...