It's great fun and very rewarding
Being very technically minded, I have 'poked' into every computer I have owned over the past 20 years and tried to understand how it 'ticked' and, in virtually every instance, modified it to suit my requirement - usually meaning improving its versatility. Having helped friends with their computer problems over many years, it became quite common for them to offer me their redundant one when they up-graded to a new one. So, as a challenge, I used to see how I could modify them to 'improve' them.
Then, one day, I was given a huge tower with room for four optical drives and head-room above the power supply for an extra group of HDDs. So, I set about seeing how much I could cram into the case and make it an 'all-singing-all-dancing' computer! It ended up combining two of my existing computers in the one case with a two-pole change-over switch to power one set of HDDs or another set. With six of these, three in each set, I had to procure a card to drive them with the result that it was the four optical drives that were directly connected to the Motherboard - unconventional, but it worked! But, of course, one can, only, connect four HDDs to a card so the extra two, one from each set, were driven from a SCSI card. In addition, there were cards for USB and other hardware as well as drives for both sizes of floppy discs. Luckily, there were about 8 slots, both ATA and PCI so I could put in all sorts of extra periferals, And, it was in the days before sound, video, Ethernet or modems were built in to the Motherboard.
So, how did it end up? On one set of HDDs I could use Win2000 and I filled it with applications, the icons filling the screen. There were upwards of 200, not all having icons on the screen but listed on the Start Menu, of course. I learned a lot getting such a full computer tamed so as not to be too sluggish! On the other set of HDDs I had Win98 and DOS as I used WordPerfect 5.1 as my main word-processor with, again, quite a lot of other applications such as Quattro Pro and similar. And it is, still, in operation and I fire it up fairly frequently, say once a fortnight, as it is used as my main back-up of e-mails. And, occasionally, I run some of the applications just for fun! So, as I said in the heading, it gives a great sense of achievement and fun. Needless to say, I have learned a lot from such exploits, even if there has been a lot of 'head scratching' in the process when things did not go right first time! Still, having been retired for about 15 years, I can use the technical skill I have acquired over the years, and have fun in the process.
But, to answer your correspondent, I doubt if one can do this sort of build and take such liberties with modern components. So, to get 'the latest specification' would be very difficult for an amateur, in my opinion. The physical construction is more involved with liquid cooling, etc. But to build a workable computer with a moderate specification should be a practical proposition. However, as you realize, I used what I had been given for my builds and have bought only a few extra, special, cards or components, such as HDDs, on eBay, for instance, so the total cost has been quite low.