This MacBook Air from 2015 tends to sell for about $1000 (check price)
128GB PCIe-Based Flash Storage. Apple 13.3″ MacBook Air MMGF2LL/A. 8GB of 1600 MHz LPDDR3 RAM. 1440 x 900 Native Resolution. 802.11ac Wi-Fi, Bluetooth 4.0. Main Features. 13.3″ LED-Backlit Glossy Display.
There’s really not a lot of upgrades you can do for this MacBook Air.
2015 MacBook Air Hard Drive Upgrades
You can upgrade the MacBook Air’s Hard Drive to a larger storage capacity, but it’s not cheap with 256GB SSD Upgrades going for about $250.00 check price and 512GB SSD’s selling for about $350.00 check price
A few other upgrades to consider would be adding a new battery (not sure you need it in a 2015 model) or adding a monitor for more screen real estate.