Upgrading Storage in Macbook Air

I've seen a few similar questions to this, but I wanted to get an exact answer. My beloved 17" MPB has finally retired due to a logic board failure. Rather than spend the money to purchase a replacement board, I've decided to invest in a new Mac. After much deliberation, I think I've decided on the Macbook Air; however, I am going to purchase all the upgrades possible for it including processor and memory. I also want to upgrade storage, but I've realised it's about $500 cheaper to upgrade everything on the 125GB MBA, and purchase an aftermarket SSD from another company and replace it myself than buying all the upgrades from Apple. I just want to verify that I will be able to do this, and that there isn't something blocking aftermarket SSD upgrades on the lower end Macbook Air.