If you're buying a used machine, you should remember to factor in that it may have a 3 year old battery and keyboard. For me personally, those are the two greatest wear and tear items.
Technical specifications for the MacBook Air "Core i5" 1.8 13" (2017*). Dates sold, processor type, memory info, hard drive details, price and more.
everymac.com
The 2017 is using a broadwell era cpu, since the ultra low voltage cpus skipped some generations. It's definitely weaker, but I would consider it for writing and compiling code.
I typically prioritize ram == storage > cpu > gpu.
In this case I haven't tested how 8GB is holding up these days. Your results may vary. I would check how much memory unity and unreal can typically use. You also need to consider what your storage requirements will be like, and you shouldn't lie to yourself on that one. No one wants to buy an ultra light laptop which constantly requires an external drive.
You mentioned game development tools. Are you also going to play games on this? I don't think you would enjoy that going forward.
Lastly you don't mention how much of a difference. It needs to be a lot less. I typically factor in the cost of a possible battery replacement on an older used machine when determining if it's worth buying.
I'll tell you from personal experience, I prefer 1-2 larger displays and a full keyboard if I'm going to be using IDEs (such as XCode). That's me though.