Hi,
I don't think you can only obtain the Win 8.1 media. If you did manage to acquire it the OEM product key embedded in BIOS would not activate the install.
You could purchase an OEM version of Win 8.1 from an online retailer. Newegg sells Win 8.1 64 bit OEM version for $99.99 plus shipping if you are in the USA.
You can remove HP secure keys in BIOS. I would advise against doing that.
I have an 810-150se with Win8.1. I removed some useless programs and changed the partitioning scheme on my system with no issues. Created a Microsoft Recovery drive (USB stick) and then saved an image of my setup on an external HDD for emergency backup.
Spending additional money to install the same operating system sans some proprietary HP installed software is not an option for me.
Hope you find a workable solution.
Jaco