Since it's your boss who's in need of the computer, I'd recommend to wash your hands of the project as soon as possible and lead them to a good builder like mbailey said. That way you don't risk your job in the future if something goes wrong, be it their fault or poor components. If they are happy, it would be polite to reward you with some nice Christmas present for your help and advice. In the opposite case your bacon is at least half saved.
I help people with computers for living, and although I can, I don't build computers for my customers. When you're buying components for just one machine, there's always one missing or faulty component and getting a replacement takes so much time and effort it isn't worth the 100. Not to mention possible breaking under warranty time. As you know, the builder shouldn't charge for his work at warranty issues...