I don’t think I could reccomend Source, unless you are intent on pulling your hair out :D
Sounds like you are going more for a games design role? I reccomend Unity in this case. Also, to make it as a game designer, you are going to have some really innovative concepts…so I wouldn’t worry about using premade assets or what the graphics look like; your ideas for a game should work even if its a really simply blocky character running around a blocky environment (which you should be able to protoype yourself). I reckon Unity is your best bet.