Re: Sailing alone??
I bought my first boat three years ago, and I sail single-handed almost all of the time (unless you count my dog). The boat is a 1966 28.5 ft Pearson Triton. It's a well known model, old but there are still many around. My feeling is that if I were doing it over I would probably go with something a little smaller. Not because I couldn't handle the boat myself, but because when you get up in that size range (say, over 25 ft?) you're likely to be looking at boats with systems on-board- inboard engine, plumbing (head and galley), electrical. For a first boat, if you're planning on doing mainly day-sailing, you may not want to deal with the extra complexity (and expense) of those systems. After a few years, if you find you're really into it, then think about moving up to something bigger that would be more comfy for say, a weekend trip.
For single-handing it's not so much the size but how the boat is equipped and set up. Having a roller-fulling genoa and winches for trimming the genoa make single-handing a lot easier. You're less likely to find these on an 18 footer, so a slightly bigger boat (22-25 ft) may very well be easier to single hand than the smaller one.
Browse the listings on yachtworld.com and sailboatlistings.com to get an idea of the boats that are out there that might look suitable (size and price), and then start looking for opinions on specific boats.