First. Komplett is a well respected company and can be recommended. I've dealt with them myself and had excellent service.
Second. The package price probably as good as you would get anywhere, and the components are all quality items.
Third. Is this kit exactly what you want? If it doesn't fit your needs precisely, it rather defeats the purpose of building your own system.
Four. And the one big thing I don't like about these Komplett kits, is the proviso at the bottom of the page, that items can be changed if the published parts are out of stock. To me, this kills it stone dead. That is not to say, you wouldn't get perfectly acceptable replacements, but the whole point is, until your delivery arrives, you don't know exactly what you are buying.
When I build a machine, it is a precise amalgamation of parts that I know will work well and will satisfy the particular needs of my client. I could not risk something that is an unknown.