VirtualBox on Linux: What Windows image to use and where to get it?

I had to just do this recently, there are many option here:
https://developer.microsoft.com/en-us/microsoft-edge/tools/vms/
Microsoft Provides them for free to Web Developers but it is a fully functional Windows OS, just choose which version of windows with which IE you want, select Virtual Box, and download.

In your case, feel free to use the IE11 on Windows81 (This is Windows 8.1), that way you can be using this for many years to come as long as you keep it.

You can install applications. The only down side is there is a expiration of 90 days, so you would need to make sure you save all your App Settings / Backup and Documents off the Virtual Machine before then, I would suggest before you even boot the VM the first time to make a snapshot, that way you can just roll back to this moment, and get going again installing your apps and transfer over your files, so you don't have to re-download or create a new VM.

The UP side is that you don't have to do any windows install or setup a virtual machine or have to create any boot disks, nothing, download, import and go!

Since you say your using Tax Software, I would imagine you could create a save location somewhere in your home directory using an auto mounted folder. Save you a lot of time when you have to re-install.