Congratz in advance! Getting your first 6* is an awesome milestone.
There was this thread here that had some pretty good advice.
To answer your question, yes, Magnus is a good choice. One of the few C speed Mages, good looking art, good stats, and you can get a mlb Magnus for about 60-70TE. I would definitely recommend her as a first 6*.
Also consider these: Paris (which is what I got), Pregare, Morticia, Matahari, Petra
They all have their up and downs, but in the end, get something you really like!
Just a reminder: DO NOT GET ALEXANDER
Wait for the next event to start, rumor has it, if it was anything like the event for getting Pregare, it wouldn't be out of the question for a free player to grind for a 6*