580ex goes for $760. I've the 580, 420 and the sigma. 580ex's recycling time is very much faster than the other flashes. Sigma 500DG's recycling is slower and build quality isn't as good, imho. 420ex isn't as powerful as the other two but it's good enough for most people.
Seriously saying, if you do not know what you are going to do, you might be better off with a simple flash until you know what you need. I have this 420EX and it works just as it should. Without me having to play too much with the setting, yet powerful enough for most shots. 80% of the time I do not use flash, kind of spoiled the original lighting or unnatural. Even with 420EX, you can play with flash compensation on your camera and the bounce/swivel on the flash itself. T
Coming from the perspective of a freelance wedding photographer, a 420ex is sufficient but a 550ex would be more ideal. Reason being the recycling time is absolutely important in capturing more (good) shots. In indoor settings, more often than not, bounced flash would be used and this requires a larger amount of flash power - the recycling time is especially important here. Too slow a recycling time would mean that you'll have to miss some shots due to waiting for the flash to recharge.
As for the lens, it depends on which body you're talking about. For a 1.6x fov body, ideal lenses would be 17-40L, 15-30, 18-50, etc. for indoor shots.
Not really more in the long run. I believe in working with constraint. There are a lot of better equipment so you can take better pictures, but does it means you cannot take good pictures with lousier equipment? Seriously he have not tried the flash he may not even know if he needs a bigger one. I believe that the thread starter already knows what he wants to buy, just need some confirmation here and there or else he will not state his budget, knowing that the 550EX and 580EX falls into this range