As others have rightfully said, any Mac the can run Mac OS will work.
Couple things to think about.
Apple just extended the requirement that apps in the store must be built with the iOS 13 sdk. If you get a used machine and plan to have your app in the store, ensure that it will run the current version of Xcode.
Also, cores are your friend with development. I have a MacBook with four cores and a Mac Pro (trash can) with 6 cores. Build executions light up all those cores, every time!
An Air will run, and it will be slow. Patients will be a good thing.
128 is tight when you add design software, etc. You will need to trim the fat off xCode with one of the utilities like Dev Cleaner. It will remove gigs of files that you may not need.
I had a 128 gb SSD on a machine, it would be frustrating to get Xcode to update as it needs the disk space head room if you are running close. Add to this problem that SSDs are no longer upgradeable even after your Applecare has expired. Most of soldered onto the board.
Hence, my advice it to stretch to a 256gb SSD, you will really appreciate it later!