This is an ambitious idea and might make for an interesting project. The first thing you need to consider though, is whether it's possible to send a craft like this out into the stratosphere. Imagine if you were flying a plane and saw hundreds of Science Buddies readers' projects floating about! You would need to contact the FAA and find out the procedure. At least a few years ago it was allowed, but you had to work with them on a schedule, file a flight plan and give them tracking information so that they could make sure that no one was put in harm's way.
What you're planning to test is a good question. What makes electronics fail? If the parts on the board aren't designed for weather extremes that could be a problem. Poor assembly methods might make a device fail as parts expand and contract. Many years ago I received a wonderful radio as a gift and I asked my father (an EE and the then-head of audio engineering at a major manufacturer) about reliability. I then learned how car radios are designed (and tested for) extremes that might kill normal AM/FM radios. It was news to me then and I've always considered when designing things.
So what to send up? You need some way of finding your payload when it comes back down. Are you planning to track it yourself? A GPS with some ability to send its position seems like a good idea. Some students do that with phones (although it's prohibited by the FCC). Will you include things that you know will fail to see if they do? Or rugged things that are designed to not fail? A laptop, especially a running one is likely to fail if not protected from the temperature. If you want to send one to see *how* it fails, that might be a reason to choose one.
Since your idea is so broad it's a very big topic to discuss in a forum like this. I'd suggest you start with a little research. What equipment do you need for a near space launch?, where do you get it?, how much does it cost?, what procedure do you have to follow to launch one? I'd get the basics down first. If it turns out that you need a huge expensive balloon to launch a full sized laptop and some tracking equipment, then maybe that will dictate what you send. Look around the web and see what other people have done.
http://jeffpfaller.com/2012/06/sending- ... near-space is a really nice site, as is
https://blog.hartleybrody.com/in-space. Both groups sucessfully launched light payloads of a camera and a tracking device. Their experiences may give you some specific ideas.
Good luck, and please write back after you've done some more reading.
Howard