You have two options, both assume you have a second hard drive installed in the PC.:-
1. Uninstall MEP and then re-install, choosing the Custom install option, and changing the installation folder to the second hard drive. Note some files will still be installed on the SSD this cannot be avoided. Then change the programs default folders to the second hard drive..
2. Leave the software installed on the SSD and change the folder locations in the program settings to point to folder(s) on the second hard drive - this will leave the software on the SSD but video files, temp files etc will be saved on the second hard drive.
I would not use an external hard drive as the second drive, the USB transfer speed is (IMHO) not fast enough for two way data transfer during video editting and playback.
. . . . i was thinking about little things that mep is going to try to write anyway in it's folder. . . . .
You do not want to go down that route unless you are very experienced in changing Windows Group and user Policies - it involves remapping the Users Appdata file and Program data folders, and you may kill other apps and even Windows if you make a mistake.