Welcome.
A lot of folks go with the Gillespie (GCI) brand handled by Rapco. A few go with the Aervoe but most use GCI. Everyone seems to have their favorite primer system too. Frequently, a lot go with local sources for primer. Lots of discussion threads on those points.
Pretty much same with stencils. Militarystencils.com (Rick Larsen) is pretty popular though I know quite a few just go to their local print/stencil shops and have them made up. Shop around. Rick seems to be the preferred.
Parts are all over the place for these. VPW as NDT mentioned probably has the largest selection in one place. ABLinn (ebay dealer), Midwest Military (John Bizal in MN), Adirondack Dodge (NY), and many more. You have the Iola show in August that you can find just about any part made for these trucks.
If you haven't already, visit the G741.org site. All about the M37 series trucks and variants. Also, take a look at the registry and see where your truck fits in. And, by all means, it is strongly encouraged to add your truck to the registry. Quite a few of us haunt both sites but the registry is an added bonus that helps us all.
Good luck with it and hope to see photos of it soon.