I may not have built a guitar before (though intend to attempt to soon) but it's clear opinion is divided into several camps here.
First you have the "Don't run before you can walk" school, which is fine, and where I sit myself. Keep it simple, plan, and if you do things without haste and for a first attempt make something simple (No triple neck with a fanned fret nine string, 12 string and bass) and have the motivation, then it will turn out well hopefully.
The second school is the if you plan it, then try something a little tricky- after all, you don't learn without attempting something. This is true, and cool enough- but not fool proof (as a professional fool, it doesn't suit me!) but for people who have some woodwroking experience and who are hell bent on making only ever one guitar, a great option.
Then there's the lazy school of just buy one and do some routing. Personally, yes, you could and it would be easier. But, doing from scratch allows you to fiddle with other stuff like headstock shape, finish etc. which you want to be unique from the ground up.
Ultimately, I think it's better building- if you enjoy building and are after a really unique axe that would normally be a custom job. If, however, you just love a plain, simple, strat with no bells or whistles and hate woodwork, then buy that. It's always your choice, and it's always a matter of preference. My thoughts are that buy building guitars you learn many useful skills that can be applied to other aspects of woodworking, painting and design, you have to create a hybrid design which must seemlessly integrate artistic elegancy and technical functionality. Few people can combine scientific and artistic creativity, originallity and function. This is one of the few hobbies that can.
Pete