new pc: in what order?

System repair disk anytime after install. May as well make it as item 6/.
I make both Windows inbuilt AND Macrium images to external HDDs for extra safety.

Windows inbuilt requires the System Repair disk to do a restore. For Macrium you should make a Restore pe disk and test that it works. You can make the Macrium pe disk yourself by following the Macrium WAIK download guide. WAIK is a large download and a member has kindly provided access to a Macrium Restore pe:
Imaging with free Macrium
The download link is in the Tutorial.

Windows is imaged using Macrium.....you dont "keep Windows on an external hard drive"

soulfood said:

Would you recommend having two external hard drives, one for windows and one for macrium images?

I'm assuming you mean windows inbuilt images versus Macrium images. Of course both are imaging the Windows OS partition.
You can keep copies of Windows (inbuilt) images and Macrium on the one ext HDD. I keep images and other backup data on 2 external HDDs simply for hardware redundancy.