I'm not OP but I've done this stuff on and off for two decades and I thought I'd toss out some answers.

About 20 years ago I got started using a dozen or so surplus Pentium75 desktops and by combining the working parts and best/largest parts I ended up with eight or so nodes with a reasonable amount of memory and local storage.

A write up would have been pointless it was pretty much standard like you'd read in any linux magazine of the era.

I used one of the PBS options for batching and some homemade stuff to rip audio CDs to mp3 in a distributed manner and completely unproductively fooled around with PVM and MPI. In the old days PVM was stable and easy to use although MPI was predicted to be the wave of the future. In fact today PVM is pretty much dead and everyone uses MPI or the ever popular NIH homemade stuff. "Back in the old days" we did things like spawn off ray tracing jobs semi-manually with bash scripts and perl scripts. You'd write perl that output animated povray files given a frame number and then queue up like 120 ray tracing jobs and then stack them into video and thats how you got a minute of bad ray traced animated movies. Not being much of an artist my animated ray tracing was limited to bad physics demos like mirrored ball bouncing. As most other cluster ops do I calculated a lot of "X+1" and "Is X a prime" to test. Like most screwing around, honestly I just turned a lot of KWh into heat.

The wife acceptance factor was incredibly low. Although admittedly I'm still married, to the same awesome woman even. Eventually I got rid of all the machines. But for a year or two it was a lot of fun. I thought the noise level was bad... then I replaced all the boxes with dell towers a few years later that sounded like jet engines. Then IBM 1U rackmount servers P4 surplus around 2010-ish which did a pretty awesome job of running LXC before containerization became "cool". Portability ha ha ha the size of a closet in the middle of my home office, looks like an organized disaster zone. Really looking forward to building a pi cluster to screw around with that'll fit in a shoebox with room to spare. OMG I could build a toy cluster so small I could drop it into my desk drawer when I'm not playing... Oh if I had more time this summer I'm feeling the urge to build another cluster...

Power doesn't matter as long as the circuit breaker doesn't pop. At one point I figured I was burning 1100 watts using the dell towers (and yes the room does get warm) which implies running 24x7 would cost me $1100/yr (which is very cheap compared to a training class or many other hobbies, but I digress) however I suspect I never powered up that cluster more than maybe 300 hours. I spent 10x more money on a then new technology 100 meg ethernet switch than I did on the power the system used. Back in the dinosaur era we mostly used hubs which were unswitched one big broadcast region, probably sounds very weird to modern young kids.

I can't even begin to list all the stuff I learned. The jump from admin'ing one unix box to admin'ing a cluster is almost as big as the jump from being an appliance user to being a cluster admin.

What I didn't learn was the scalability problems of having 10000 nodes, having less than ten generally, I did a lot of setup by hand. Although I did all the OS level stuff automation like homemade scripts, puppet, etc. A lot of cluster admin is related to the "fun" of physical hardware support for 10000 individual boxes and I miss out on that. Also running 24x7 how do you handle dusty air filters or whatever at that scale for day to day operation? Power failure recovery must be non-trivial.

Its hard to say "well I'd never have gotten job X except for..." because I might have anyway. As a hobby its been tangentially tied in with my day job on and off since at least the turn of the century.