Hmmmm.... I definitely would not recommend playing in chronological order; the official Feel my soul score is very tough, I haven't been able to work out how to do it!
Before you even try to play any song you like, start off with a simple guidebook and get your alternate strum tactics down. My music teacher friend who's familiar with J-pop says that YUI's strum pattern is fantastically faithful to the pace of the song.
As already written below, it may take up to a year to learn how to sing and play simultaneously, but don't give in. Choose a song that you really like, but doesn't have tons of barre chords or worse, barre chords all at the first fret.
At this time of writing there are three songs from YUI that require you to tune down your guitar by half a semitone: TOKYO, I can't say and Skyline. You may want to concentrate on songs that don't require this preparation unless you have two acoustics lying around.
There are four songs that require a capo at the first fret - Summer Song, It's My Life, Your Heaven and HELLO. Green a.live needs the capo on the second fret.
Some songs have chord patterns which have a very obvious sound match to the singing part - YOU is the best example. Some don't seem to have any resemblance - HELP is one of them!
I recall that you consider LIFE to be a favourite. The official score for that is 'barre heavy' though and may hurt your hands a lot - it does for me!
I've just finished photocopying all the new songs from the HCYL scorebook, even the ones I don't have plans to learn..... maybe I'll upload it later.