Lots of good advice already, like buy the biggest lathe you can afford. Unfortunately in my part of the country there isnt a very large market in used machine tools. I have had the Grizzly 7x12 for about 4 years, and like another poster I have and still am modifying it, mini-lathe.com and littlemachineshop.com are good pages to look at to start. I also looked at Sherline machines, including a bit of hands on with one in the university art metals studio. One thing that steered me towards the Grizzly was the availability of 3rd party tooling and accessories, a lot of stuff is available from regular industrial suppliers and some things like morse taper tooling you may be able to use on a larger lathe if you move up where Sherline, Tiag and similar companies tooling seems to be pretty specific to their machines. I have done some stuff like working titanium on the mini lathe I didnt think I would be able to do. Someone said they consider these mini machine tools as kind of a kit, you are going to have to do some work and tinkering, but with some care and creativity you can turn out fine work. Someone mentioned ebay, I look there to watch the market and pricing, but I would be very leery of buying a machine that you cant look at in person.
Todd