@sumudu I would recommend the best level of specs you can afford, as time goes by, a lower-specced machine will lose efficiency more rapidly. Examples are the two laptops I bought in 2011: one is a 15-inch MBP that's been retro-fitted with an SSD instead of the slow Hd and 12 GB of RAM; the other was an 11-inch MBA, with a 128 GB SSD and 4 Gb of RAM.
The kicker in this case is the RAM & the MBP's extra dedicated graphics card, that keeps it still relevant as a computer to this day. Whereas I've disposed of the MBA because it's 4 GB of RAM just wasn't coping well any more.
I replaced that MBA with a used, two-yr-old ex-Apple owned 11-in MBA. It had been used by an executive in Sydney and, as an Apple-supplied/owned device, had the highest specs available at the time: 512 GB SSD, 8 GB RAM, the 1.7 GHz Core i7 processor & Intel HD 5000 graphics card.
I'd recommend a13-in MBA (greater battery life than the 11-in), at least 512 GB SSD & 8 GB RAM.