Minimum requirement to start app development

Not really.
You might be able to use a very old version of Xcode...
...but this would be so far behind the current state of the art, that you would struggle to find relevant resources, and would be unable to deploy to recent devices.

I would advise using the current version of Xcode (14), and getting a mac (possibly used) which is capable of running that.

Xcode 14 requires macOS Monterey or above, so you're looking at:

• MacBook Pro introduced in 2015 or later
• MacBook Air introduced in 2015 or later
• MacBook introduced in 2016 or later
• Mac mini introduced in 2014 or later
• iMac introduced in late 2015 or later
• iMac Pro
• Mac Studio
• Mac Pro introduced in 2013 or later

What robnotyou said plus:

• 8 GB is clearly not enough. Need at least 16 and 24 if you have large projects to develop (figures may be lower with M1 Mac).
• At least 1GB storrage
• A 17" screen (an external monitor of 21" or 24" is really a nice addition if you use a MacBook).

All in all, don't buy any model older than 2017: it would rapidly be obsolete.

If you want to publish on the Appstore, you need a recent Xcode:

As of April 2022, all iOS apps submitted to the App Store must be built with at least Xcode 13 and the SDK for iOS 15.

Which means macOS 11.3 at least (Big Sur). High Sierra is 10.13.