I have some insight into how not to do it. :)

https://www.reddit.com/r/tifu/comments/2ncv6i/tifu_by_accide...

https://ukhas.org.uk/ Has a lot of very good information on the balloons people use along with the type of radio transmitters which are legal to use in the UK etc.

I did a flight like this but using Arduino instead of Raspberry Pi (total overkill and power hungry). Full details: http://blog.jgc.org/2011/04/gaga-1-flight.html

Looks more efficient .. with the use of Arduino

Yes [0] Shoot me an email. I'm happy to answer questions.

[0] - http://robotrising.org/StratoSphere/collection.html