Since it sounds like this is your first sewing experience at all, you're probably better off starting with something you can find an exact (or really close to exact) pattern for. Pattern modification can still be tricky even for people who have been sewing for ages, so if you can focus on just sticking to the instructions for the first go-round, you'll save yourself a lot of headache!
Soren's outfit it at least pretty loose-fitting, so you would have some give on the lower part of the tunic and sleeves. The top, however, is a little tighter (would require some tailoring), and the detail work is definitely going to take time no matter which way you approach it.
Since you mentioned Sailor Moon, maybe one of the girls' school uniforms? There are definitely patterns and tutorials out there for those specifically!
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- Emperor Lelouch (Code Geass): 4% (Planning) Otakon 2015
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- Pyrrha Nikos (RWBY): 1% (Planning) Otakon 2015