You don't need to buy a new hard drive to load WIN 7. Just make a "full" system image backup of your current WIN 8 install to an external HD. An excellent FREE backup and restore app is
AOMEI Backupper. Or you can use WIN 8 built-in backup. Just make sure to make a self-booting recovery disc on a USB flash drive or CD/DVD.
Next, go the the manufacture's website for your specific computer, and get any WIN 7 device drivers you may need. Some companies make this easy. Toshiba had a single file which included all necessary drivers when I installed WIN 7 on one of my laptops.
I would then recommend deleting all partitions on the WIN 8 HD so they become unallocated space. You should be able to do so with Disk Management, and will complete upon re-boot. You now have a fresh drive. Formatting the drive itself will not do this.
Windows will create new partitions and do a full format(surface scan) of the HD before installing WIN 7.
Is all this worth $40? I would think so, but you could do it yourself. And if too much to handle, take it to the tech at a later time.
BTW, WIN 8 does have some user friendly features built-in that can reset your system to a fresh install state. This is convenient if one installed corrupt device drivers, programs, or even if malware caused enough damage making your system unstable or unusable. It's called "Refresh Your PC" and restores WIN 8 to factory defaults. It doesn't affect your personal data files, but you will have to re-install any programs or applications you installed.
A second option is a System Reset, which wipes out all data and installs a fresh copy of WIN 8. This makes it easier if you want give your PC away. But should follow up with a secure free space wipe, one pass is all that is necessary, using an app such as CCleaner.