there are barebone laptops that you can assemble yourself, though those always come with limited choices in terms of hardware and tend to be more expensive than the pre-made alternatives.
but in a majority of cases as USAFRet has mentioned, there's simply no universal standards between laptop manufacturers aside from RAM & HDD/SSD. a motherboard on an Asus isn't going to have the screws and ports align perfectly with a Toshiba casing, if it fits at all.
unless you're an enthusiasts/hobbyists, the best choice here if you value your money is a prebuilt notebook. most desktop pc builders mock prebuilts not because of some blind devotion that they should only run on something they assembled with their own hands, but because it is overpriced most of the time. it all comes down to the bang for your buck, and in a notebooks' case, especially for someone like your dad, sefton who i assume will NOT be gaming or doing any CPU intensive tasks, a $500 i3 notebook would be more than enough with the added benefit of a warranty for the full set rather than individual parts.