Long term effect on Mac computer of installing Windows/Running a Windows VM I need to install Windows on my MacBook Pro. Hardware limitations are not a problem for me. I am mostly concerned about the long term effects that running Windows might have on my Mac.

I am assuming that there will be no impact on the hardware. If I install and run Windows for the next 6 months, and then uninstall it, then the hardware would respond exactly as it would 6 months ago. Is this true?

My biggest concern is regarding what might be left behind due to this adventure. I will be either using Virtual Box, VMware or BootCamp. In any case, I will use an external drive for storing either the Virtual Machines or the partition.

When I don't need Windows anymore, does deleting everything on the external drive mean that all traces of the Windows activity will be gone, with no noticeable effect on my Mac? If not, what will remain?
There will be zero issues running Windows on a Mac.  A Mac is basically an Intel based PC. 

If you are using a VM, regardless of the platform (VMware, VirtualBox, etc) once you delete the virtual machine, it's gone.  Unless you share folders between the VM and the host, there is nothing that goes outside the VM container.

If you are using BootCamp, once you delete the BC partition, Windows will be gone.