I am aware that Macbook Air can't do heavy work. That's why i am looking for a Pro. The price for the one a friend of mine is selling is 600euro with upgrade Ram and SSD.
It depends on what you mean by 'heavy', an air is more than capable for 99% of web dev tasks. I work in firefox os which includes needing to rebuild gecko constantly, as well as the occasional full android stack build and the macbook air is sufficient.