I'd get as many as you can afford now, and add more later. I don't see the ST-E3 being worth it until you have at least 2 600's. The great thing about canon's RT system is you don't have to buy an extra accessory (i.e. pocket wizard) every time you want to add a flash to your setup.
I bought my first 600 at the refurb $373 price, and I got this 33' cable:
In anticipation of using it off camera right off to bat...I have an umbrella and manfrotto holder on the way too.
I figure I can do this, and save and buy a 2nd 600...and have dual flash...cord connection to first one, it being the master to the wireless 2nd one.
After that, buy the ST unit for 100% radio control.
Dang...really hating that I didn't get that 2nd 600 unit when I had the chance before...still, I have a watch notification set up for the refubs when they come out again.