Also would you mind explaining what the vehicle harness is? And what did you mean by swap everything, lock, stock, and barrel?
The vehicle harness being referred to is "all of the wiring" in the vehicle. In the manufacturing process, the wiring is all pre-made by a vendor. All the different colored wires are pre-cut to length, terminated in the various plugs, sheathed in tape and wire loom material, labeled and ready to install. Then Ford (or whoever) lays that harness into the vehicle, usually near the beginning of the assembly process so that it is ready for all the components - lights, engine, battery, air conditioning, computer, radio, etc. to be connected as they are installed. Depending on the donor vehicle, and what swap you are doing, you may need various amounts of that wiring for your swap. Most of the time, you will need the entire engine compartment portion at the very least. If swapping from a really old vehicle to a really new vehicle, you may need a lot more because of the changes in vehicle technology over the years.
Swapping "everything, lock, stock, and barrel" means complete engine, transmission, transfer case, wiring harness and ecu. Doing this as a whole set ensures that you have components that are compatible with each other. Just getting any old 2001 4.0L RBV* engine with any old 2001 RBV transmission with any old 2001 RBV transfer case and any old 2001 RBV ECU and wiring harness does not guarantee success, especially concerning the ECU. Different components require different parameters and options to be programmed into the ECU and you need the right wiring configruation for those options. Again, the newer the donor vehicle, the more important this is. And we haven't even mentioned the possibile necessitiy of fabricating special parts that sometimes comes with these projects. Stuff like accessory brackets, engine mounts, different length drive shafts, etc.
I don't want to be negative. I usually like to encourage everyone to try new things. But an engine swap is not a good beginners project. If you are "not very well versed in mechanical work", I would recommend that you buy an older vehicle in running condition that will require you to learn gradually and build your skills, knowledge level and toolbox inventory before tackiling a major project.
* RBV = Range Based Vehicle (Ranger, Bronco II, Explorer and their counterparts in Mazda, Mercury, etc.)