Computer building (assembling parts) is not for the inexperienced. It does require some skills and some knowledge. The actual build will take 4 to 6 hours but the study process to learn building will take in excess of 100 hours.
Regarding warranty matters the usual rules apply. If the component has manufacturing defects then the manufacturer's warranty applies. However, as the previous poster stated, if there is user error then you are responsible. This is outlined in most warranties.
$100 is not a high price for assembling a computer. Just make sure that the person doing the assembly is competent, does not take any short-cuts, and has a good reputation in this field.