I need to write working requests but I cant test them on our live systems. My idea was to run a local instance of Magento and test my requests there. Is that a good idea? What is the easiest way to do that? Any help is appreciated.
I can recommend AMPPS for a fast webserver setup, but of course you can also use MAMP, XAMPP etc.
Afterwards, you can either copy your live system to your local host or install the required Magento version from source. n98-magerun2 is a handy tool to install a specific Magento version using the command line.
I'd say that the easiest way is to create a copy of the live website files on the webserver along with a duplicate database and connect the two.
If the duplicate Magento instance is within a subdirectory of the live site (e.g. www.example.com/duplicate), you will just need to update the base urls accordingly.
I don't think that this is the best solution, but if all you need is a non-production website to test against remotely, I think that this would fit the bill.
You'd want to be sure that there is enough space on the server, and it would be a good idea to lock down access so that you don't attract confused public users and search engine indexers.
Running a local instance or a dev instance on a separate server are better solutions, and it is common to do both. But they require the ability to create and configure a webserver, database, install the nec. PHP version and extensions, etc. Depending on the nature of your testing, this might be overkill.
Whatever the solution, I would recommend removing customer & order data from your test database.