Interface USB and SPI flash (Boot PC from SPI)
You can't boot from anything that BIOS doesn't supports. As basically your BIOS supports USB, HDD, ODD, Network boot - that's your only options.

As for the write protect - just get a USB flash drive with write protection switch. These are rare nowadays but still could be found. And finding one is much simplier than building your own "SPI USB". Especially for "not an electronics guy".

Actually that you are trying to do is to build your own "USB flash stick" with a slower serial interface between some sort of the USB controller and SPI memory. It would be pretty big project and I doubt the efforts are worth your goals since there are ready made products which just do their job already.