There are no stupid questions in our hobby, my friend...only arrogant, condescending answers. None of us knew anything until we learned it; can't learn if you don't ask.
A used, stock wiring harness like you already have can be successfully retrofitted...IF you have a pretty good understanding of how all the various EFI components are supposed to work, and good reference manuals for both your existing vehicle and the EFI system donor vehicle, to chase everything down. There are certainly excellent reference manuals readily available on both subjects.
Painless Wiring makes high quality stuff and has good instructions and tech support. Not inexpensive, but a good value to the builder who isn't a whiz at electronics.
Windsor-Fox probably has the best, almost turn-key setup for your particular retrofit application...and also the highest price.
Bottom line, ya' gets what ya' pays for with any of these choices. None bad, just different.
Since you already have a stock harness, you might see if you can find an experienced EFI auto technician who you could hire "on the side" to help you when you get stuck. You might be able to pay a couple hundred bucks for the help and still come out well ahead. The peace of mind that would come from an experienced "consultant" checking out the installation is worth something in itself.
Steve Amos