I want to buy a Western digital Hard drive

yes a WD drive will work fine with your Mac. I have one myself.

but please make sure that you don't use any software that is preloaded on the drive (or install such software yourself) that supposedly make a drive "Mac ready". when you use software from just about any manufacturer for their storage device you run the risk of that same software not being compatible with macOS updates / upgrades. it can cause the software to stop working. at best this will cause you a headache. at worst it can render your data inaccessible. it is always best to use disk utility to erase and format a drive before use to remove any preloaded software.

please see Erase and reformat a storage device in Disk Utility on Mac - Apple Support

Yes, it will work fine with your 2014 MacBook Air. 2 GB is a good capacity for using Time Machine with that Mac.

if you want to use the drive for Time Machine (for backup) and for storing extra data, you are setting yourself up for disappointment. once you assign the drive to Time Machine, that drive will become "read only" the only thing that will be able to write to that drive will be Time Machine itself. you will not be able to add anything else to it.

if you want to have a Time Machine backup and also a means to store extra data, you will be best served by getting two or more drives and dedicate each for it's own purpose.

and as affordable as external drives have become, there is no reason to not use more than one drive.

when it comes to external SSDs, I prefer the SAMSUNG T7 SSD. IMHO, they provide a good compromise between price and performance.