Currently there is no noticeable difference between gen 5 and gen 4 as an OS drive for general use, this is well known.
You need to remember if you have an SSD and simply buy a new SSD and install a fresh OS, it feels faster and snappier. This leads to a lot of people thinking they feel the drives performance.
Certainly aim for a B850 motherboard with a Gen 5 NVMe slot and Gen 5 GPU slot, I think currently B850 is the best performance per pound with less of the PCI lane gimping we see on X870/X870E motherboards. I feel the same way regarding Gen 4 NVMe, speed is not noticeable, yet double the capacity per pound with some brands, I went with Samsung 990 Pro's simply because the Samsung 9100 Pro cost twice as much and offered me no noticeable benefit equal to the outlay. This may change in future if the Samsung 9100 Pro drops in price.
Your Crucial P510 is £88, the equivalent Gen 4 is the P310 at £62, so at £26 difference not really worth caring about unless you can't finish the build, I bought 990's over cheaper Gen 4 drives as I have had good experience with Samsung and like the Magician software.
It's a hobby, your allowed to spend or waste money on odd choices, I wasted £100 on two MicroATX cases recently that turned out not to be suitable, also £36 on a CPU cooler that changed it's mounting hardware, and bought a bunch of fans to try, for a PC I am supposed to be stripping down and getting rid of. Meanwhile I bought the same cooler and fans for a temporary case I am currently using for AM5 as I cannot decide in what direction I want to take things.
The case and PSU look great, I'm sure your lad will love it, it's similar to what my own lad is currently speccing for himself, though his build is based on the NZXT H6 in white, I am trying to get him to watch a few case reviews 1st and telling him not to get hung up on Youtube Reviews of CPU's and GPU's. But ultimately, his money, his choices.