I got the 2014 MBA for R12 000 last year this time.
Its been a great buy.
I still have the original OS and Windows 7 on it, so I can't say how El capitan and Windows 10 will run on it.
The 4 gigs of ram has been good enough, I was also concerned about that.
I don't think you can upgrade it.
Mine has a 128 GB SSD, which has been big enough so far. I don't store my media files on it, if I go away for a weekend or so, I may copy a few movies etc onto it.
If you need more space you can always get one of these:
http://www.takealot.com/transcend-jetdrive-lite-130-for-64g-macbook-air-13-inch/PLID32842810
You can get an extended warranty for it which you can buy just before your one year warranty expires, so don't buy it on day one if you don't want to.