Here's a short article on what to look out for when buying a second hand Mac, be it a Macbook or iMac...
Storage - that affordable, light Macbook is affordable for a reason! 128Gb storage just isn't enough for a laptop that you are using for work. Most of the storage space is taken up with the OS X itself and the apps you run for work like Adobe Creative Suite and Office applications.
Try go for at least 256GB storage and make use of your cloud storage for the data you generate, keeping the hard drive clear.
Memory - 4Gb will make you grind your teeth with frustration as you wait for your machine to do something. 8GB is a standard amount for older Macbooks but if you are doing more than browsing or light office work, then 16GB will be better and the faster speed will let you get on with work without thinking about your Macs performance.
Processor - older Intel processors are still great but you'll find that you won't be able to keep up with software updates as they are now designed for M1 and later processors. That means a slow degradation of the ability to run newer software or update your OS X version.