I learned to sail on one when I was a kid, and had one too as an adult. Just an amazingly fun and easy boat to sail, while at the same time, it's suitable for experts as well. The boat is quite the hot rod and will plane with little effort. As much fun as she is to sail, there are some things to consider if you are considering buying one. The boat is wet; in any kind of chop, you will get wet. And since the boat is really easy to tip, you will likely get fully dunked overboard. And to get the most out of it, you need to be aggressive, which means a lot of heeling (see above) and hiking out. I've found that as I get older, I find it harder and harder to hike out that much. The abs of steel I had at 16 seem to be a thing of the past. Someone above described the Sunny as "car-toppable". I think that's technically true, but not really practically. You need two strong people to handle the Sunny, especially to get it high enough to put it on and take it off the top of a car. Not for me. I think you realistically need a small trailer if you are going to move the boat more than once or twice a year.
Having said all that, you should be able to find one in good shape for about $500. That's a lot of fun for a little bit of money. There are also a number of Sunfish clones out there, the Phantom being one of them. If you aren't going to race, you don't need a class-compliant Sunfish, so you can potentially widen the field of potential boats to include the clones.
Have fun!
