Does anyone have any advice on how to go about building this? I've considered trying to base from an ODST armor kit, but I'm not sure how to properly modify the kit, or if it'd be easier to build from scratch. It's for an OC
Have you considered the Helljumper class Gen 2? It's the in-lore version of ODST armor that former Helljumpers wore when they merged with the Spartan program. It's pretty much what Buck wore in Halo 5.
In that case, yes, Helljumpers is it. And in reference to your post in the Armory, it is the full ARMOR. However, it is not sliced for pepping. You can import the obj files into Pep Designer and do it yourself, if your willing to work. Or someone might have already done it, or be willing to do, but you'll probably end up at the end of a list of projects.