how hard/expensive would it be to make a computer/laptop that is trivial to take apart/reassemble?

yes the current mac pro tower is great for access to internal HD, PCI-e cards memory and optical drives.

Does anyone know of any brand of PC case that is anywhere near as easy to swap parts inside? I’ll happily pay a premium for it, 4U rackmountable models especially.

My thermaltake m9 is nice