The main advantage of buying a commercial kit is speed.
If you havent got the time, everything is done. But it is done to whatever standard the company dictates.
You can always change thing later, but you have a kit to start with.

However that is not the way I would do it.
Making the kit yourself has many advantages over buying a kit.
-You dictate the standards of the equipment and supplies.
-You learn about each bit as its added and how to us it.
-You pack the kit, so you know where to find things when they are needed. (Hopefully <img src="/images/graemlins/smile.gif" alt="" /> )
-You havent spent money on something that wasnt up to standard to start with.
-You have a kit designed for you and your environment.
-You will probably end up with spares, or enough items to make up other sized kits as you go.

Disadvantage : Cost