I've got an idea bopping around in my head for making an NPC. I suppose for a game this old it may not be the best idea, and arguably there are enough party joinable NPCs already in existence.

Barring those problems & perhaps others, I'm taking a look at how Kelsey is put together as a guide; I'm hunting around "his" site to find some tools I'll need, although if anyone wants to give me a specific pointer I'll happily take it. Beyond that, any comments to on how best to proceed?

Just the obvious one: don't make him overpowered!

The one thing you generally hear about new NPCs is that they're hideously overpowered, come with the best weapons and abilities in the game and can pretty handle every enemy on their own without taking a single hit.

True. One should take very much care to balance an NPC. Other than that, I would make sure he has a good background story and some nice interactions, Kelsey-style if you wish :)

Learn WeiDU for the purposes of actually coding the mod. http://www.weidu.org

Barring those problems & perhaps others, I'm taking a look at how Kelsey is put together as a guide; I'm hunting around "his" site to find some tools I'll need, although if anyone wants to give me a specific pointer I'll happily take it. Beyond that, any comments to on how best to proceed?
Because of how old Kelsey is, and the fact that he wasn't originally developed as a WeiDU mod, Kelsey-SOA is a poor coding guide--and the code files will tell you so.

Believe it or not, but my Ghareth one-day NPC is actually a much better guide, and aside from a romance it does all of the "core" things you'd expect in an NPC, from start to end. A few things are omitted, like slayer-change commentary, but for just about everything else, from the joining to the epilogue to the talk-fixer script to audio, it's the route I would suggest.

The one-day NPCs are at http://www.pocketplane.net/oneday