Should I get this for the complete version I have on GOG (christmas and secret files downloads) or are the fixes already applied?

I played it using this a few years ago but it was still pretty buggy on the PC I used then.

It depends on how you feel about purity.
Personally, I've been using the J2R but I should mention it isn't 100% feature complete.

Well, JJ2+ is more a patch to the existing engine, whereas J2R is a source port.

So what features are J2R missing?

The changelog shows. For example, the freeze effect works, but the art for it isn't there.

JJ2+ is well worth getting. It adds features, fixes bugs, it's basically required for online multiplayer at this point...

GOG provides a download with Plus pre-installed, but you can also install it manually.

Very good point. If having JJ2+ installed and seeking for online multiplayer games, you can simply join the JJ2 multiplayer server to stay in touch with the whole active multiplayer community. https://discord.gg/VDgSFdpgSH