You'd have to balance it out between scripts/features/etc and time/progress/work. Best thing to do right now is to plan the game out right now, more specifically the scripting part. As for engine, stick with rpg ace for your first game. Once that's done you can get started on the other parts of making your game, ie art, story, so on. I've been there before, always looking for scripts and always trying to add more stuff. It's an ambitious undertaking, but definitely not the only important factor in making a game. As i mentioned before, make a list of things you want in-game, and work from it through that. As for making a quick game, i think that's a great idea. Making a non-time consuming and small game is a good source for feedback and results. Whether it turns out good or bad, it'll be valuable experience for the 'big' game you're planning later on.