These have been shipping with 240W power supplies - bear in mind they're small and use a proprietary 6- or 8-pin connector to the mainboard - you'll be very limited in upgrading later and will need a third-party adapter cable to connect a standard power supply.
PCIe and NVMe are synonymous.
It looks like 16G is the max.
Since you're already looking forward to upgrades - the XPS system might be a better starting point. The very small case the Inspirons come in limits things as far as future-proofing.