You might be interested in
Rewired (doesn't seem to be on sale tho?) for input since the old Unity input system is very lacking, but the new one that's still in preview is getting pretty good so Rewired might become irrelevant soon. Worth looking into!
Unsure about the A* plugin since Unity has pretty decent navmesh stuff out of the box, especially with
the GitHub repo with extras. Might wanna read
this discussion (this talks about Unity Pro for some reason but what I'm talking about applies to free Unity where I've used navmeshes before so I'm a bit confused about that) before making your mind up. If you're just getting started with Unity you should probably just see how far the vanilla system takes you anyway. I'm sure there will be more sales down the line! c:
Gaia does look cool! I've barely tried Unity's inbuilt terrain system because I just don't make the types of games that need it, but what little experience I've had has been underwhelming, so this looks like it might be a great investment if you're going to be working with that kind of stuff. Would love to hear a comparison from someone who's more experienced with it!
Good luck with Unity and games!