It's easy to do if your mechanicaly skilled a little, it's one of the easyer jobs becuase you don't need to remove the timing belt. if you can remove a wheel and change a few bolts rembering where you took them out you can do it.
try searching for a guild how to remove the timing belt as you'll be doing the same in the start. then once your as far as removing the first belts ect just remove pullys and replace with new ones then fit you new oversized belt inc with kit job done.
It'd be a pretty straight forward job mate. Like jimm said if you've got some basic mechanical knowledge it'll be a doddle. It says that comprehensive instructions are provided so as long as you know how to follow instructions you should be fine.
on a stock engine u mite be lucky to get 1bhp but it does help and once u start adding lightened flywheel, crack pulley, cam pulleys ect you really remove alot of engine inertia what means faster revving engine. you mite not gain much bhp but you will get power faster and a quicker throttle response.
anyone correct me if i'm wrong but i hope that helps.