Does a Boeing 747-830 exist and if it does, is there any difference to a 747-8 (748)
Until 2016 Boeing included customer codes into the aircraft model names. Your example 747-830 decodes as follows:

747: This is the general aircraft series.
-8: The 747-8 variant (in this case, a 747-8I, where I is for Intercontinental, the passenger variant).
30: The customer code for Lufthansa or Condor Flugdienst (see e.g. Wikipedia for a list of customer codes).

So this makes it a Boeing 747-8 produced for Lufthansa (Condor does not have one).
The term B748 is the ICAO code for the 747-8.