Just take our build that we recommend for you and take it down to a mom and pop computer shop and have them build it for you. As to weither or not he's trustful for what he offers, I don't know nor do I care as I typically deal with people in person whenever possible.
Or read up online how to do so. In general after part selection all you have to do is discharge yourself of electricity by touching something like the metal case and than drop parts in place (if it fits it goes there, and things are color coded, changing oil is 100x harder in a car) and than instal the OS which is pretty much putting in the disc and following the instructions and answering a few questions. Installing drivers is easy to, you have the part numbers just go on website of the company who made it and find the product page and find where "drivers" are it is typically under product support or downloads and download and instal the one for the OS you just installed.
And in general pre-builts are crap. They will only offer certain things on certain models and are greatly overpriced. I can typically build a so called "high end" machine that will be as powerful if not more so for about 1-1.2k and have it be more powerful than whatever dell, alienware, etc can cook up that will cost you well over 4-6k. As an extreme example. They also have a habbit of trying to pawn off low end graphics cards as "gaming cards", mid range cards as high end, etc. And will greatly overcharge for overclock things, where as now days if you don't care how bad the OC is you can just use the motherboards that will overclock for you.