Most people who build a H3 marine use the Army Cut Uniform Universal Camo Pattern (commonly referred to as ACU). While this looks ok, and is a cheap uniform to find, if you take a closer look a the camo pattern on a marine's uniform from Halo 3, you'll notice that it's not the digital pattern found in ACU, but more of a geometric or tiger stripe pattern that closer resembles the Airman Battle Uniform (ABU) pattern used in the Air Force. ABU pants and shirts can be found on eBay for a pretty good price.
So, essentially, if you build a Halo 3 marine pilot, this is what you'll be aiming for...
http://media.moddb.com/images/members/1/400/399748/Halo_3_Pilot.jpg
Even the black drop leg panels can be found on eBay (search for "drop leg molle black" and you'll find something that looks similar).
The only real challenge to this build would be the chest plate. I had neglected to mention that earlier; but it might require a bit of scratch building, or you could simply use a standard marine chest plate.
As an added bonus to this, you'd have a relatively unique outfit. I've only ever seen one or two marine pilot builds in my years here.
On to your second question, "are ODSTs simple?" I would say yes and no. While they are not nearly as complicated as building a spartan suit, they still wear a lot of armor; armor that needs to be scaled correctly and strapped sufficiently. So, building ODST armor can still be a challenge.