Heading down the Speedlite off-camera lighting path...help before I get too far

Hello,

I am on the cusp of heading down the speedlite off-camera lighting path and would like to survey the collective wisdom from the board as I start to make my initial purchases.

I am a hobbyist photographer that has a day job and simply does photography because I really enjoy both getting better at it and the accompanying joy that results from those that are the subject of my work. I am currently shooting a Canon 7D but anticipate upgrading to a new camera within a year. Over the last year I, using borrowed gear from a friend, have started to do several flash off-camera shoots – senior portraits, families, etc. All of this work was done outside (e.g. I don’t have a studio, but I do have a basement). I have been using my friends 550EX, stand and umbrella for over two years (!). I recently bought a YN560III and a set of manual radio triggers (RF-603) based on input from this board and plan on using that for most of my staged shoots (e.g. stationary subject). Well, alas, after two years it is time for me to get some of my own gear.

The YN560III is a good manual flash but I need to get a TTL flash that will replace the 550EX that I can call my own. So, I have two goals and I am hoping those that have my more experience than I can help me as I start down this path.

1) Goal #1: I need a replacement for the 550EX that will allow me to do TTL lighting. That is, that one flash that I will keep on my camera when “walking and popping”. Of course I will also use this off-camera as well as need.

Some options I am considering: a) 430EX II - $200 on eBay and has TTL capability; b) 580EX II - $300 on eBay and I have now added a master; c) 600EX-rt - $400 refurbished and will allow me to expand as my strobist approach expands; d) YN568EX - $180 which has TTL but no master.

2) Goal #2: Make a purchase that will help me walk the fine line between getting what I need to learn as I cost effectively as possible VS a set-up that will allow me to expand without having to buy news sets of gear (as opposed to small incremental adds) as my hobby grows. Do I bite the bullet and make sure the centerpiece is a good as it gets (600EX) or do I go on the cheap and build as I go (YN568EX) or in-between (430/580)?

My budget is about $600 and I will also need 1-2 stands, 1-2 umbrellas, and perhaps an off-camera TTL sync cord (3ft and 30ft). Obviously with the more expensive flash solutions I will buy less of these extras but with the cheaper flash I will be able to get more of these extras, hence the dilemma...

Thoughts?

Tom