Lowest budget way to build iOS apps from this link (that are not in the app store)

I am looking for using open source software as much as possible. The posted link is a nice source for open source software for the iphone. Unfortunately there are a lot of apps you have to build. I read that you need mac for this. I own an iphone, ipad and watch but no mac mini or apple laptop. I only have linux and windows. So what would be the lowest budget way to buy a device where i can build those iOS apps? I guess a refurbished or used mac mini? What are the minimum requirements? Do you have recommendations?