For a quick reference, check out CascadeWalnut's factory SMB build in the following thread
and ask your self if that looks hard or not? Will you have the ability to keep your wire lines neat and tidy, organized and labeled so that you know where things are being hooked up?
I wouldn't necessarily say that it is 'hard', however it is time consuming and you need to be fairly meticulous to ensure you do a safe job. I think if a person has the time and some reasonable skills that it can be done; however you've got a few questions that would lead me to believe you still have a bit more homework to do before jumping in:
Is it necessary to buy a wiring diagram? Nope - you are MAKING the wiring diagram. You need to have this planned out. Figure out where your wiring needs to go, determine how long your runs are and what your electrical demands will be and size your wiring and fuses accordingly.
Fuses / fuse box? YES! I'd recommend a main fuse/circuit breaker off the auxillery batteries as a main disconnect point / connection for inverter, and then a distrubution fuse box to run your accessories out of. Blue Sea makes a nice little one.
Where are your batteries going? What type of batteries? If the batteries aren't sealed, are they in a location that allows proper venting?
I'm certainly not an electrician or a mechanic but am currently working on wiring my own van. It's tedious work, but rewarding when you flip a switch and the light comes on! This is definitely an area that you want to feel comfortable with your work though, as the consequences could be very serious if things are not approached properly.