I got a new 2013 11" MBA exactly three years ago. I still use the original operating system, 10.8.5. Everything has always worked fine, I have the i7/8gb/512gb model and sometime run demanding software like Final Cut Pro, Logic Pro, VectorWorks, Strata 3d CX, Photoshop, etc. These are all old versions, so there would be no advantage to running a newer version of MacOS, and I subscribe to the "if it ain't broke, don't fix it" philosophy.
If I were buying a 2013 machine, I would play around with it first using whatever version of the operating system is already installed. If you like it, leave well enough alone. Otherwise, upgrade to a newer version, there are plenty of threads here from people who say the newest versions run fine on these machines.
Now, regarding the SSD. If you want more than 128gb, then
don't buy a 128gb machine. Upgrades are expensive and there are various threads here with complaints about problems from some vendors. Buy a machine that already has the RAM and SSD capacity that you need.