Upgrade to a hackintosh?

I have had the opportunity to purchase an affordable bold Mac Pro, but I want to strip the insides and put in a new graphics card, SSD’s and CPU, however, as it is a legit working Mac I would have access to OS X. As I am very new to this and don’t want to invest unless I can make it work. So my questions are if I keep the original hardrive with OS X, and replace the internals with new stuff, can I update all the way to Mojave? What would prevent that from happening?

If I can make this work I would use it to upgrade my iMac as it’s very much past it. And if the OS can keep being legit, would I be secure going forward?

Thanks for the help!