Suikoden I and II is worth buying a PSP IMO unless you have to beat the first and then carry on to II.
I'm not *exactly* sure what you meant by this, but there is something you can do with 1 and 2 (and 3 as well, but there's no point in doing it with 3). If you have save data from the last save point in 1, you can import the save data into 2 to get some extra dialogue, as well as a few other extras.
And no, you don't have to play every one of the games to understand the whole story (there really isn't a 'whole story', like I said, because Suikoden is really just a chronicle of the 27 True Runes). There are, of course references in 1, 2, and 3 to each other, but 4 has no references to the others.
The Souleater is the first True Rune that the story tells about in Suikoden 1. It also has the most powerful spell set in the series so far (IMO), not to mention that it's just so freakin' cool to begin with.
I didn't like 3 mainly because after playing 1, which I thought was good, and 2, which totally blew me away, I had high expectations from 3. I was expecting voice over, a battle system at least close to the original, and *at least* some likeable characters. I didn't like Hugo and I absolutely despised Chris (but Geddoe was really cool. He should have been the main character!). The data port from 2 to 3 was total crap, and I hated how the story jumped around so much. That's why I didn't like it. Also, I hate how they give you the 'name your own character' feature. Every game so far (except 4) has had a main character that actually had a name (Tir from 1 and Riou from 2). That was probably the only thing I didn't like about 4 was how they always refer to the main character as 'him' or 'you' or 'he' and so on.