Do not go into this thinking you will make any money. Yes you may be able to sell the boat for more than you bought it for, but you will pay for every upgrade with way less return.
My advice may differ from most but I would look for the cheapest boat that comes with the most stuff. Stuff equals money. Those extra sails, shore power cables, flare guns, life sling, fire extinguishers, canvas, seat cushions, tools, all add up to thousands of dollars you are not spending. Make sure when buying the boat you get everything.
In your price range stick to craigslist and local marina postings (go walk around the marina and look on message board around the marina, plenty of boats for sale), and only spend about half of your anticipated budget, I would suggest maybe even a quarter of your budget. You should know things like where you are going to keep the boat. Your marina may only have 25' slips available, so that means a 28' boat wont work for you. It is way to better to know this stuff a head of time so think about your options.
If you buy a boat for $5000 a would suggest using common sense and a friend from a local yacht club vs. a survey, another thing is most boats have a semi recent survey if you just ask or look through paper work. You want to make sure that standing rigging and running rigging are intact and that the boat has a complete sail inventory, semi-recent bottom paint is always a plus, and I also enjoy expense and service records. Have fun, and when you find a few boats post them here and everyone will gladly give you their opinion.