I'm planning to buy MacBook Pro 13" 256GB SSD but i find it too expensive. I thought that it would be better if i buy 128gb SSD and upgrade it with an HDD storage. Is that possible? If not, is Apple still selling the old MBP?
The latest MacBook Pro 13" (MacBookPro12,1) comes with a special internal HD interface based on PCIe and uses a special form factor for the SSD. Check the ifixit replacement guide how it looks like. This replacement guide shows a MacBook Pro 2014 but the SSD in the recent model looks similar.
So internally there is neither space nor an appropriate interface to upgrade it with HDD storage.
The old Macbook Pro is still available on Apple Store.