Going by your budget and sold prices on eBay in Europe. The best models I think you can get at auction with at least 8GB RAM and 256 GB SSD are the 2014 MacBook Air and 2014 Macbook Pro 13 Retina. Although it will be easier to find a 2013 model for that budget from better known sellers.
XCode versions are heavily tied to OS X versions. Originally it came as part of the operating system. It was an optional install but it was apart of it. Now it is an optional download but Apple still pairs it with the current OS X version. It started out life as "Developer Tools" then was rebadged XCode with Mac OS X 10.3.
Technically, you can run XCode 1.0 on a G3 iMac with 128MB RAM running Mac OS X 10.3. So, that is the minimum requirements for XCode. If you want to run the current XCode 9.4, which I assume you want. You will need to run macOS 10.13 High Sierra. While the beta version of XCode 10 runs in High Sierra. Odds are the final release will require macOS 10.14 Mojave.