To fully appreciate the evolution and experimentation that has defined the series from the standpoint of the console RPG experience, I think you'd best be served by playing them in order. You'll also gain a better appreciation for the elements that reoccur in the series (Masamune, airships, etc.) since you'll have a reference for their origin.

The difficulty at the beginning of the series (if you are a purist who wants to play the games in as close to their original incarnation as possible) is not really related to the character building or combat system. You will definitely "get" the later character and combat systems better if you start with the earlier games because, generally speaking, the later systems build on the earlier (and offer their own twists on them) and you will recognize what abilities, spells, summons, etc. have been useful through the series. Where the early originals lacked in player-friendly information, you can now readily find more than you'd ever want to know with a simple Google search, so that's not so much of an issue anymore.

Finally, I've found the earlier games generally take less time to "complete" (unless you're grinding to max out stats or trying to pick up a 0.000000000000000000001% drop /shakefist @ FFIV). The later games have more side features and quests that, if you find yourself getting into the game, you will feel obligated to complete and will themselves take up several evenings of playtime. In short, if you start with the earlier games, you'll probably get more of a taste of the series quicker than you would playing one of the later.

At any rate, I would recommend at least IV and III and/or V before playing IX, as IX serves as sort of Playstation-era "tribute" or recognition of the first five or six games before the series moved on to the PS2, and you won't appreciate the "nods" in IX as much without having played the early games in the series.