Hi Andreas,

Some had success on free software by selling the executable of their application, as it is in itself a commodity for Windows and MacOSX users, may also work for GNU/Linux users too.

You may also offer executable with some limited access to your application.

Not sure it will turn to be successful if you offer additional services against a fee as you described:  it will be an additional strain, distracting you from developing your application.

Best wishes of successes

Hilaire

Could this be something to start working on a new kind of "Steam" for free software games? One that does not simply sell an executable and not tell you anything, but the opposite it will give you the source code and then sell you education for developing for it and offer tools to buy/sell your very own art?

-- 
GNU Dr. Geo
http://drgeo.eu