“If You Fail to Plan, You Are Planning to Fail”
Benjamin Franklin.
And so what was true in Benjamin Franklin's time is true today. This could be an incremental project as money can be found, upgrading one component at a time, or it could be more of a big bang project, assembling all the parts you need and then one weekend put it all together and build it out. But there are several things you need to plan for
1. Firmware updates
This should be the first thing you do. So if your donor machine came with at least some RAM (even if the wrong speed longer term) and a hard drive, and boots up, don't go throwing things out just yet. You will need to update EFI levels before you do anything else.
2. Initial Operating System
If your system didn't come with a hard drive, go find an old one. You probably have something that you threw in a closet and forgot about, or have a friend who did so and can borrow an old drive from them. Next, partition the drive as a GUID partition and format it in Mac OS Extended: The file system used by macOS 10.12 or earlier. You should install a fresh copy of OS X 10.6 or 10.7 if you are rebuilding a 2006~07 Mac Pro, 1,1 or 2,1. You should install OS X 10.10 or 10.11 for your 2009~12 Mac Pro 4,1 or 5,1.
3. Purchase New or Used Components
This will include a Metal capable Graphics Card if you are planning to run OS X 10.14 or later and at the moment, thanks to crypto miners all graphics cards are crazy expensive.
It will include SSD 3D NAND for you to replace the spinning pieces of metal that make up a hard drive.
It will include new CPUs and RAM
It may also include additional cards for USB3, etc.
And you may need to replace anything worn out - noisy fans for example
