Best Phone For Up To 400$ ?

Hi all,

I'm looking to replace my old, buggy and barely functioning Samsung Galaxy S.
The budget is up to 400$ and the key features I want are:

1) A fast and responsive deice(Good CPU, RAM, Touch response etc.)
2) A good camera
3) Not too big: Glaxy S3 size is the maximum, an iPhone 5 size is great.
4a) A popular model so that online support & accessories are easy to access
4b) I think It has to be Android or iOS for the same reason as 4a.
5) this is not exactly a feature but I will point out that I live outside of the US so I need an unlocked or at least an unlock-able phone.
6) New phone (not used).


So far I think the Galaxy S3 is a good option, but I'm open to suggestions.
Also, if I choose to go with galaxy S3 - which one should I get? there are several models and so far the i9300 seems the best from what I checked out.

Thx in advance, ZBBZ.