Modding-wise, what do I do now?

[Ewokgod]

I think that before I make any more suggestions, I should know the engine better.

A while back, Solarius Scorch linked me a couple of sites.
https://www.ufopaedia.org/index.php/Ruleset_Reference_Nightly_(OpenXcom)

and

https://github.com/Yankes/OpenXcom/blob/OpenXcomExtended/Extended.txt

I think that I understand some of this enough to dabble, and I believe that I have a decent little project to try to implement. It's just a trade good for sale to the black market. My only question now is;

What program should I be using to edit the files with? Is Notepad++ enough?

[Meridian]

What program should I be using to edit the files with? Is Notepad++ enough?

Yes.