Desktop. They are expandable, cheaper to build, if hardware goes bad easier to fix. Literally the only thing a laptop offers that a desktop does not is portability. If you need to move it laptop, otherwise desktop.
His point was a desktop you always have the option to upgrade down the road if you see fit. With a laptop you are stuck with the hardware you order. What happens when he gets a new DAC that doesnt have USB but optical only? SOL unless he has a desktop that he can swap cards in.
Ive built tons of custom RCA cables. Mostly for car audio applications with Neutrik ends. Cat5E cables work fine in a car audio environment were there is almost always more interference with signal than in a home application.