I would say the costume is definitely two pieces, a jumper and undershirt.
If your costume is going to be a separate shirt and jumper then I would say it would make the most since to put the zipper on the back of the jumper and have it close going upwards. But if you are set on putting it on the side then it should definitely close downwards so that if anything the clasp doesn't catch on your sleeve and rip anything.
As for the shirt sleeve it looks to be all one piece with the part you're talking about being a bunch in the fabric rather then a separate sleeve itself. If you want it to look just like the picture though you will need to do two separate sleeves as otherwise you will need to make the sleeve a bit too long and push part of it up under itself to make it look like that.