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#11
If you decide to buy a Small SSD (120G) I can guide you to move the C:\Users folder(SSD) to D:\Users (HDD).
This is a good practice as all the trash (on C:\Users\NAME\AppData\Local\Temp) moves to the HDD.
Edit: You don't need a new Win 7 license as you will be using it on same computer, just changing disks.
As you plan to dual boot (win 7 and Win 10) my suggestion is install them on the SSD as UEFI - GPT (can also guide you).