Trying to make my 1st custom Elite Hideo Itami, any tips?

I would personally try to find something that already has the black shorts/trunks like the Aries you mentioned. You could paint them onto something like a DB figure that's just wearing regular trunks, but unless you're an expert at sealing the paint, it's going to be a bitch trying not to chip near the joints it if you were to move the legs. Regardless of what you end up using, you'll want to seal any paint you apply anyways. I use Krylon Matte Finish. Tape off any areas you don't want sprayed, like the head/torso/etc. and spray it on in light coats. As for decals, you'll either have to make them yourself using decal paper (I like waterslide decals) or there are people who sell them.

Good luck!