I'd say start with FF7, since that has the most documentation on everything.
FFVII was already implemented earlier. Problem that FFIX and FFVII has almost compleatly different collision and walkmesh move detection system. FFVII a lot easier. But FFIX more advanced... it seems.
FFVII work almost directly. Itf you press forward came calculates all collision and if it can do the movement it do it. FFIX has separated walkmesh movement and entity collision system. Still don't know why they need such thing.
Do you still have a todo list of what's done, and what isn't? I'm interested in contributing but I'm not sure which part I should start on
I have one. Don't know if it is up to date.
Mainly things that needs to be done almost from scratch:
Camera system (implement 2d and 3d camera, add exporters for dat dumper and battle).
Sound system (implement sound in qgears engine and write dumpers for sounds and music in FFVII).
Movie system (implement system, implement camera for movies).
2D Background (i'm working on understanding how FFIX backgrounds work and implement it in game. Shader based depth sort).
Scripting system (delayed until LUA 5.2 is out)
Entity system (character controller and scene load)
Effect system (particle system and dumper for battle particles)
UI system (implement effect based UI, add exporter for blender, recreate ffvii UI)