A computer is heavy, fragile, expensive to freight and easy to buy almost anywhere. Sourcing it locally also means local warranty and a local shop that can swap it in a day, which matters far more than the purchase price when the counter is down on a Saturday.
What is worth bringing: the small things that are hard to find or oddly priced where you are trading - a spare receipt printer, scanner, cash drawer, cables. They are light, they fit in a suitcase, and having one spare of each is what turns a two-day outage into a ten-minute swap.
The all-in-one advice is the one people push back on, because an all-in-one looks neater on the counter. The trade-off shows up on the day the touchscreen or the built-in printer fails: with separate components you replace the failed part locally that afternoon; with an all-in-one you replace, or wait for, the whole unit.