What's the extent of customization in Skyrim for the PC?

Yes to all five of these questions.

The stats limit thing requires some clarification: some stats have a bound above which they give no benefit (like armor rating or spell resistance), some are bound by variable size used to store them (You can't have a million billion hitpoints, but you can have several hundred thousand).

Changing and adding NPCs is one of the least important aspects of modding, you can change or add locations, graphics, gameplay mechanics, et cetera.

All mods on Skyrim Nexus are free, and so are the mods in the Steam Workshop. I am not aware of any sites that require payment to download Skyrim mods; though there might be some scammers around trying to sell mods, the mods they'll try to sell will likely have come from a free source.

You can mod pretty much everything and its all free.

This is an old list but gives you a good idea of what's possible: The 25 best Skyrim Mods

And there's always Syrim Nexus