Yeah, like many other people have already said, try downloading them first, that way you won't be disappointed.
Though, to be honest, I find I love the CDs a lot more if I've never heard them before and just choose a random one. That's how I fell in love with RAINBOW when it came out. Then I did the same with I am... and Memorial Address. It's 20 times more fun when you buy the album without having listened to any of it first.
But I digress... XD Anyways, regarding her earlier works, I guess it depends on what you consider earlier. I consider Duty and earlier to be her early works, but that's because I became a hard core fan the day after RAINBOW released.
I definitely suggest holding off buying A Song for XX before you download it. That's a tough album for many fans to get into. See if you like it first. Any of the other albums I highly recommend. LOVEppears is more trance and dance pop. Duty is sort of dark rock pop. I am...is soft rock pop. RAINBOW is extremely hard to describe; it's very... mystical, for lack of a better word. Memorial Address is dark. So I'd just pick which music genre you like the best and try that.