Macbook Air (13-inch, 2017) - should I upgrade or refurb

I don't think you can upgrade the RAM on that MBA (not sure if you can do it on any MBA). It's soldered to the motherboard.

According to this article (https://ssdsphere.com/upgrade-ssd-in-macbook-air-2017/), you can preplace the SSD hard drive, but you need to use some kind of adapter, as Apple use a non-standard connector, and you can only go to 512GB. I doubt doing so would result in much of a performance boost (it's not like you'd be going from a HDD to SSD), and anyway, you could free up some space on the current drive with a bigger external SSD like this: https://www.amazon.co.uk/SanDisk-Extreme-Portable-1050MB-Dust-Resistant/dp/B08GTYFC37 - for roughly the same price, and certainly less hassle. That and a wipe and re-install of the OS is about all you could do. But I wouldn't expect amazing performance gains - it's the RAM and the processor that's holding it back.

Looks like CEX will give you €191 for it

https://ie.webuy.com/product-detail?id=slapappmba7295b&categoryName=laptops-apple-mac&superCatName=computing&title=apple-macbook-air-7-2-i5-5350u-8gb-ram-256gb-ssd-13%22-osx-b&referredFrom=boxsearch&queryID=416572d4cdf2e681a2c1beb4820f8bd6&position=4

This covers everything: https://support.apple.com/en-us/HT201065