Is it possible to choose a position to scan or its only centered at the rocket-silo ?

Not really, I know how to make it, but I have not done it yet.

For example: set on the map a marker and the zone will be scanned

yes in such a manner would be nice....

could be also exceptional expensive like say 50 radar, 1 satellite, some blue cards for data processing....
say it would be a satellite-package with additional fuel and so on

like in the manner of a single use satellite with the pointer device attached

not at least make some research into this ?

You can find such thing in the https://mods.factorio.com/mod/FogRemover - it adds geostationary radar satellites on every rocket launching.