NAV 2018 do have really a short lifecycle that would expire in January 2023.
You could purchase application builder and solution developer granules (and spend quite a lot of money) to make use of the old C/AL test toolkit but I would say this is a pure waste of money.
With Dynamics 365 BC and AL development this is free of charge and you can develop your automated tests even with customer licenses.
In my humble opinion, it worth sparing all the money that you would like to invest in creating C/AL automated tests for a version close to be discontinued (windows client is already deprecated, fyi) and use them in upgrading to the latest version.
You will gain more feature, more robustness, a modern client approach, longer lifecycle... and so on
