My i3 with 6 G RAM 8 years old machine(came with Vista installed) is running fast on Windows 10 with normal HDD. Your system should run fine. What antivirus are you running ? Antivirus is a major cause of slow down.
Just upgrade your HDD to Samsung SSD instead of investing money on Win 7 license if you are still not happy with performance and clean install Win 10 creaters update on SSD. Use your current HDD as a back up external drive using a HDD case.
Samsung Evo series SSD will be enough for daily usage