Thought this might be useful summary for the one or other:

Parallels Desktop10 + Windows Software(incl. License) + XDP…

Windows Software:
There are multiple choices of Windows Version, which one to choose is own decision.
I went for W7 Home-Premium 64bit.
And because my iMac does not have any DVD-drive I decided to buy an online download version.
The download version (.iso file + license key) costs usually less compared to a DVD Version. In addition I considered an OEM version rather an Original-Version, which also is very much cheaper in price without disadvantages or limitations (at least in German this is common practice and legally allowed).

Parallels Desktop10:
I downloaded Trial-Version first for to test the package and bought license later.

Special Tipp:
For Windows “.iso” files there are multiple sources available to download it.
If you make a note about the exact version of that downloaded “.iso” file (e.g. Home-Premium, 64bit), you can buy a license-key from the multiple choices in web later.
This way you can download Windows, Parallel Trial, XDP Trial and install everything to check if this works fine and satisfies your needs and only then buy the licenses.

Installation:
There Paralles Webpage has quite very good descriptions available how to install everything and how to install a windows-based software. So you may check there.



To Consider:
Some topics did not work fine for me and this is why I had to dig into the details to find a good solution. I will list this here to help those, may facing similar issues:

- XDP 32/64bit: Even so my iMac is a 64bit machine and even so my windows is 64bit too, the XDP10 did not open as long as I have installed it as 64bit version. Only once installed 32bit version, all worked fine.
- MagicMouse Scrolling: When using XDP10 (or any previous version) I normally work with a mouse, having left/right button and a middle wheel. We know from Windows, that scrolling with the wheel, let the pages scroll up/down easily.
But with my iMac I do have no longer such a mouse. I have the Magic-Mouse only. Unfortunately, the scrolling of pages did not work properly or simply by accident in the very beginning of program-opening only. (I checked Internet and learned this is known general issue and not only related to XDP in this set-up with Parallels)
Solution: Hardware > Mouse & Keyboard > Uncheck Smooth scrolling.
This worked fine for my machine and set-up.
- MagicMouse Middle Click: Another issue is the fact, that the MagicMouse from it`s nature does not support the middle-wheel action of my standard windows mouse. As a result I could not click middle button (mouse-wheel) hold it and use it to move the page in XDP around. This was critical for me, because moving pages around in XDP only with the side bars is very annoying for me.
Solution: BetterTouchTool, a small, free of charge Mac-App which seems to be THE standard for any Mac-User to extend the Magic- Mouse/Touchpad etc. capabilities.
I installed it and set in preferences middle-click as mid-button simulation. Very easy donw and it works perfectly now.