Needing a cause

I've been re-reading Samuel Clarke's cosmological argument. Here's a version of his argument:

1. Anything that has a cause needs a cause.
2. The sum of things that each need a cause needs a cause.
3. Anything that exists and needs a cause has a cause.
4. The cause of a sum of things that each need a cause is outside of the sum.
5. There is a sum of all caused things.
6. So, there is a cause of the sum of all caused things. (1-3,5)
7. So, the sum of all caused things has an uncaused cause. (5,6)

I think the trickiest and most interesting thing in this argument is (2). I suppose the intuition here is that you're not going to get what you need simply by piling up needy beings.

Where I have "needs a cause", Samuel Clarke has "is dependent". That makes his analogue of (3) trivial, but it makes his analogue of (1) more controversial.