While you may have some trouble finding a dealer who will install the 180 tire without also installing the kit, the kit is not necessary, especially if you have installed detachable sideplates or bags. The main components in the kit are new inner fender braces. They are basicallythe same braces used in detachable accessories.
A lot of guys have sucessfully installed the wide tire by just making sure wiring under the fender is secured and protected to avoid rubbing.
Here is a list of the components in the wide tire kit and the individual piece prices from Zanotti's HD.
43197-04A Dunlop Screamin' Eagle GT502 180/60B17 Blackwall Tire 17" Rear $172.66
40502-06 Dunlop Tubes 17" Inner Tube 160/70-180/60-17 Center Metal Valve $20.24
43101-06 Dunlop Rim Strips Fits 17-inch Center Valve rims $3.48
59514-06 Rear Fender Inner side plate replacement bracket 2ea $9.87ea
4592 Screw 2ea $3.18ea
10065 cable strap 2ea $0.45ea
Hope this helps.