first time painting

Awesome! Guess its time to hit the dollar store then....

Sounds like a good idea to me, you can get some practice on the cheap dollar store figures rather than possibly mess up the figures you're wanting to customize. I've done similar things myself, while in the process of working on my very first custom I ran into some difficulty with how to proceed, so instead of potentially screwing up the work I'd already put into it I started a second piece with a figure I wasn't all that precious about in order to work out some ideas.

Painting miniatures has its own set of frustrations and lessons, but it'll still help. I think painting little figs will at least help you judge how much paint to brush on. For example, if you're glopping it on, you're going to lose details.

You'll most likely be using bigger brushes on MLs though.

Edit: Just saw you said 5-inch figs, that's fine. Use figs with less articulation so you'll be less frustrated with paint rub there.

Buy some crappy figs at the dollar store, or just buy an actual ML figure from ebay or whatever, they're only like $7 shipped for the figs no one likes (Batroc, Sentry, etc.). At worst you can toss it aside and use the parts later when you're more confident.