>>7248536Tamyia would probably be a good place to start for military stuff especially 1/35 armor and 1/72 scale aircraft they might be lacking in smaller details but fit great and are reasonably priced. As for mechs I don’t know what your taste is but a kind of obscure table top called dust tactics/warfare has some kind of neat looking tank mech hybrids. There is also Warhammer that has things like sentinels, dreadnoughts, deffdreads, and Killa kanz to name a few.
It’s also probably best to start with a cheap no name brush like master airbrush for just figuring out what’s what of course it’s not a terrible idea to start with something around $100.