What type of voyage are you contemplating? The teen's background? Your budget? There are boats designed specifically for single handing (racing, classe 40, open 40, or open 30...
Akilaria Class 40 ) While they are very seaworthy and can be handled by a very skilled and athletic sailor, these are NOT beginner boats, and they are expensive. Abby Sunderland tried in one with an unfortunately predictable result.
Jessica Watson, more sensibly for her program, used a modified S&S 34
Ella's Pink Lady
and Zack Sunderland, a modified Islander 36
There are many boats that can be made suitable for the purpose you listed, but what's troublesome is that you're looking to invest in a major asset for a teen with what appears to be very little knowledge of sailing and yacht design. I'm hoping the teen you're handing this over to has a strong sailing background. If not, a 40' boat is well beyond any reasonable expectation to safely single hand, yet alone be able to maintain while at sea, or help prep for a voyage. There's a lot involved. Boat choice, while very very important, is only the first step of a very complex process. Let's put it this way... there are very few teens around who are equipped mentally and physically to be out in a 40' in 15m waves and 50kts of wind. It's not impossible, but improbable. Something like sailing from California to Hawaii is very different than sailing in the Southern Ocean, and would very much affect boat choice. You'll notice that both Zack and Jessica sailed in boats less than 40'. Remember that loads, gear, and sail sizes effectively double for every 5 feet of boat length. Sometimes bigger isn't always better or safer. Again, if you can be more specific about your goals, you'll get better answers.