Nice to meet'ya. I do think it's strange to look to Final Fantasy for furthering your Japanese interests. Much of the series especially FFVII and every game before it are rather western in tone and content.
I imagine most the early games are easy to come across. Instead of playing the first two games in their original format may I suggest Dawn of Souls for the GBA? It pretty much improved on every aspect of the original two. I assume since you've played III for the DS you'd be open to playing IV's DS re-release?
The first two games in the series can be beaten in about a day or two. FFVII and Tactics are my suggestions if you're looking for a game that's both long and rewarding. Tactics has challenging gameplay, an especially well-written story, and a rather active modding community.
Stick around.