This is an awesome idea! I would check out different Youtube or Deviantart tutorials on how to make the pendant for the necklace out of polymer clay. I've made several accessories this way when I couldn't find anything pre-made that was accurate enough. It's a cheap material and a little goes a long way, it should be something that you can work with until you get the desired result. For the chain, I've seen thick links like that available by the foot at Army Surplus.
For the tattoos, you may want to consider designing some "stencils" that you could use as guidelines for painting on the ink. Since they aren't very vibrant, you could probably get a henna art kit and experiment around with that. Doing it by hand would be extremely time consuming, which is why I suggested the stencils. If you choose to search for the "stick-on" kind of tattoos, try to find something consistent in style and shade or you'll end up looking blotchy and strange. See if you can get a friend to help you out.
The gloves should be easy, probably just a set of black tactical gloves with some studded matching bracelets should do the trick. The cap looks to be a black military cadet cap (eBay has plenty), and you could probably hand-paint the ninja star logo with some fabric paint. Try tracing the pattern on with some chalk first, that way you can erase your mistakes. If you're looking for something a bit more legit, you could probably find a local embroiderer that will take the design that's on the cap and embroider it on for you but I'm not sure on pricing and if it would be worth it to go that route.
The pants should be easy enough. For the shoes/boots you could probably just modify a pair you already own, or pick up a cheap pair at Academy or a thrift store. You can use fabric paint to make the yellow parts accurate, paint on the BSAA logo on the strap, and then throw in a pair of red shoelaces.
Just some suggestions, stuff that I would do if I was attempting this costume. I think it'll be really fun and if you get it done I want to see photos! I'm working on Parker Luciani (beach outfit) right now