I would recommend going for one of the Singer Machines. They’re pretty straightforward and if anything isn’t clear there are good tutorial videos on the Tools page (see link above).
The overlockers give a very professional finish but can be a little overwhelming to get set up and can be a bit less forgiving in their operation. Probably overkill for a costume.
I hope your costume making goes well and if you’d like to post pictures when you’re done it’s always nice to se what others are making. 
Happy halloween!