If I were to recommend a starter Koei game for someone, it'd honestly be the original ROTK. It's got a lot of options, but it doesn't have a lot of options-within-options (if that makes sense), and once you get the hang of the basic feature set you've pretty much got the gist.
OTOH, the best way to win at ROTK is to basically break the game by fcnzzvat sver nggnpxf naq fcraqvat 1 tbyq cre ghea ba guvatf (ROT13), so maybe it's not the best choice.
But I still find it has a super-addictive quality that, so far, has only been matched by Genghis Khan II out of the Koei games I've played.
BTW I find it helps tremendously in ROTK to have a text file with notes that I update as I play -- it makes it much easier to keep everyone and everything straight in your head. If the thought of doing that makes someone say "That's pretty much the opposite of what I want from a game", then maybe Koei games aren't for them.