Assembling a computer is not much more difficult than building Ikea furniture. Keep your work area clean, ground yourself, and attach the parts. It's fairly foolproof now, just don't force any parts, if you have it lined up right and in the correct position it will slide in. This site is good for verifying compatibility, use it as a checklist to cover your main components.
Software (windows) is a separate purchase. It should be straightforward as well.
This build will give you a good few years of upgrade potential as long as you're not trying to go bleeding edge. But that would be jumping to a new class of equipment.
This should give him plenty of space to upgrade disks, memory, or experiment with over clocking and cooling without risking expensive components. But each of those would be new purchases of components, such as upgraded coolers, etc. Storage would be the only real supplemental upgrade