I'd vote for used
OK, I'll start the debate. I'd vote for a good used boat. You'll get a lot more for your money, especially if it's well equiped with all the add-ons such as a dodger, autopilot, etc. Don't underestimate the expense of equipping a new boat. On the other hand, us diehard used boat buyers need someone to buy the new boats so we have something to move up to in a few years. Also, if you plan on weekend cruises, think about moving up to a Catalina 27 or even a Catalina 30. I'd bet that you could get a very nice 30 for about the same as you would pay for a new 250. I've gone from a 21 to a 25 to a 27 to a 30. I should have definitely skipped the 25 and possibly the 27 (except it was a great boat). T If you're hesitant about the 30 because of lack of experience, go ahead and buy an inexpensive, used Catalina 27 or possibly a used 25 (but unless you're going to trailer, I'd go for the 27). Save your money for when you have some experience. With regard to marinas, I'd look for one with ready access to good sailing (long motors are the pits) with lots of sailboats in the size range you're looking at. If you're sailing a 25-30 footer, a marina full of 50 foot power boats isn't what you want. Also, check the marina out on a nice weekend, are there lots of owners out sailing or working on their boats? As to preparation, the only thing I can think of is a good bottom painting with barrier coat. Everything else can be done with the boat in the water. If you end up buying used, have this done if the existing bottom is over a year old. Do it while the boat is out of the water for the survey (get one if buying used).
