A warranty completely depends on what you can find. Most of them are about $3k, for 3 years/36k miles. Which means you then have about $17k for the car. Which is enough to get a good condition Boxster.
Personally, I would look at a car that is older with lower miles. Because then you will be able to avoid CV boot repairs, 60k mile maintenance, etc for a while. The newer 986's really only have aesthetic changed, and a few HP. As a, "toy," it probably doesn't matter which year you get, unless you planning on autocrossing.
If you really concerned about an engine failure I would not get a car newer than '00. If you go pre-'01 you don't have to worry about IMS failure. But you will still have to worry about RMS, but it can be detected ahead of time. So if your missing the extra HP look for an '00 S.
Also, make sure to get a good PPI. Good luck!
