The second one will be free as well, it isn't even in beta yet. All micro transactions (I'm sure all the visual ones, not sure about stash tabs but I think they would be as well) will be usable in both games as long as you're using the same account.

Path of exile is a wonderful experience, I'd suggest watching nyxvellum on YouTube, he started playing path of exile as a Diablo 3/4 player, and has good takes both on starting blindly and using a build guide.

Don't be intimidated, don't listen to the trolls and detectors, you can play the entirety of the campaign without 2 clues to the game mechanics. End game you might find more difficult, but by then you've experienced a bit of everything and might be interested in pursuing something different from when you started playing.

If you're going to start path of exile I'll give you 2 tips, 1) get a generic basic super open loot filter (use one from the path of exile website and it'll tie to your account and be updated automatically), 2) elemental resistances (to their cap) and HP are the most important stats, chaos resistance and damage being second most important.

It's a wonderful game with a very active community and development studio.