@smet
We hope to find somebody that sells such balloons. Otherwise we will need to construct ourselves (perhaps with some help of people that have launched balloons before), which could be diffcult.
We've planned to do a test flight with a drone (up to 650m), but maybe it's not a bad idea to investigate a test flight with a smaller ballon.
@ardly Track the position, check where it comes down and pick it up. At least that's the plan at the moment.
@wvmarle Thanks for the hints. We have these issues in mind. At the moment we are wondering what technicality to choose.