Welcome to SoR!
I understand your apprehension about building something like this when you don't have much experience- however I would advise against buying something pre-built. One of the beautiful things about the $50 dollar robot and its tutorial is that it is geared towards a total beginner. Building it yourself will provide a wealth of education and experience that you can't necessarily read about.
The skills you'll need to employ to build it and the concepts behind *why* you are doing what you are doing are covered in depth on this website. For things like soldering and there are great links to tutorials for such skills. Add to the mix that the $50 dollar robot is one of the most affordable bots to build and its extremely forgiving to mistakes (i.e. if you burn out a part, it will be a cheap fix) and you have a great springboard into the world of hobby robotics.
Finally, if you have any questions while going through the process, the forums will be your best friend. There are literally hundreds of people that have built that robot here, and I'm sure any issue you might run into you can get answers for, because I'm sure someone else has run into the same thing
All of that being said, if you still want to buy it prebuilt I'm sure someone will accommodate you; but please consider what I have said.