Few technical questions regarding rocketery+minisatellites

Hi all,
as a new user and enthusiast about space exploration I have stumbled upon few questions regarding my idea of launching a small satellite into space.

First the main IDEA:
launch a small traceable unit with camera(possibly live video), GPS and some sensors as high as possible.

The unit
I know that its not hard anymore to build small PC like unit to record telemetry, take video and tell operator where it is and where its going to land. I found few off the shelf solutions capable of it which are also quite cheap.
Now few problems I found:
GPS - can anyone tell me how high can you go without loosing signal?
Radio/Antenna - how high can you go to be still capable of basic live data transmission? What to install to have live video (of some sort)?
Sensors - Is somebody working on some sort of autopilot? Not only aerodynamic stabilization but also a computer driven? I know it can be done by bunch of sensors and a lot of testing...

The rocket:
I'm not rocket engineer, nor I have facility to build or operate them, so I would need to find some enthusiast within my area (CZ or EU) which is different topic. One solution is to buy a rocket including reusable motors and use it as a test vehicle.

Tests and sensors:
What type of test do do with this setup?
What type of sensors would you mount on it?

For me the biggest goal would be to get really high and get some data back...

GPS Limits, 18 Kilometers in altitude and 515 metres per second.

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