İ am about to buy an old macos which has high sierra 10.13.6...Can i use this macos to have valid builds to publish ? i mean would i have any problems about xcode versions ? is it important for apple the version of operating system or version of xcode ??
You would not be able to submit apps to the App Store with macOS 10.13.
Currently you need to use Xcode 13 to submit apps to the App Store. Xcode 13 requires macOS 11. Pretty soon this requirement will be bumped up to Xcode 14, which requires macOS 12.
You listed macOS as a tag. With macOS 10.13 you could create Mac apps for people to download and install on your own site, bypassing the App Store.