Building your own, will often most likely always be cheaper than buying a pre-assembled, ofc there can always be exceptions to this rule
while the idea might be daunting to do it yourself, it is honestly not all that difficult when doing background research and following proper guides
depending on what country you live in, and the retailer, the reason alot of people still buy an assembled system, even when factoring in price and "ease" of building self, is the "ease of mind" and possible less hassle warranty, since its 1 warranty on the system as a whole, where as if you build your self you have to deal with a warranty per component basis
the best thing todo is to look at what your boy want/"needs" the computer to be able to do,
check what pre-build systems you can get that can do that, and then compare what you could get taht same system for should you assemble it yourselves
usually you can get the system cheaper doing it yourself, or a better/more powerful system for teh same money, but again, there are sometimes exceptions, but usually not often
first determine the needs, then decide what you are prepared for, and if/or the cost/hassle is worth it in regards to pre-build/self-build
even for a first timer, a first time build can be very easy with proper research done and guides followed, not to mention step by step video guides as one assembles, so in all honesty the "fear factor" should be the least of teh concern, consider want/"needs", price and "hassle" first before weighing in the worry of assembling self