I made a full 172 rep a couple of years ago. You will need front bumpers, wings,side skirts, rear bumper, arch liners, front and rear.
Other things you may need to include are front upper grille, lower grille, fog surrounds, fog lights and headlight washer blanks/washers
Some people have managed to cut down a 172 front bumper to fit onto standard wings but i believe this can be a pain.
Spacers, I would recommend 25mm front and 10mm rear and make sure they are hubcentric spacers. You do get what you pay for with these
With regards to difficulty it really isnt that hard to do. I had never done any mechanical/cosmetic work on a vehicle before but with a bit of common sense, alot of swearing and plenty of WD40 you shouldnt have too many problems.
One thing i would recommend doing is getting some replacement plastic arch liner screws as they are near enough impossible to remove without breaking.
hope this helps