Hope this isn't going to offend anyone, but if the one and only purpose of a retro system is as a tool to run old software, then it truly is pointless, as you can do it just fine with PCEm, DosBox or other virtualization software. So if the only goal you are aiming for is to be able to run the software, then go that route. Much cheaper, fewer frustrations etc.
For me, a large part of building and tweaking old systems, is because I enjoy the hardware side of things. Today's hardware is a bit samey. If you buy a motherboard, your choice of CPU is pretty much set. And there's only one really meaningful choice when it comes to brand of graphics card... "Is it a GeForce or a Radeon?" Asus, MSI, EVGA, etc... It doesn't REALLY matter. They're all pretty much the same.
Back in the 90's, you could buy a motherboard, and you might have half a dozen brands of CPUs to choose from... Intel, AMD, Cyrix, UMC, IDT, Rise etc. That's not even mentioning the Cyrix derivatives from Thomson ST, IBM, Texas Instruments. And for graphics card? Yikes... Matrox, Tseng, S3, Nvidia, ATI, 3Dfx, Cirrus Logic, ARK Logic, Avance Logic, Paradise/WD, Number Nine, and the list goes on. And I've not even mentioned sound cards.
Does it make "practical sense" to use a retro system compared to using DosBox if I only wanted to play games? No, obviously not. Could I do the same things on a SBC? Mostly, yes.
But it's the fun of trying out different hardware that I never had the opportunity to do back in the day, and the fun of testing different weird configurations, and benchmarking... In short, retro systems are a hobby, not a tool.
So what usefulness does the PII 450 offer? It's great if you want to build a '98 build and see what the best of the best back then could do. This is something you can't do with a Pentium III 800, as that didn't come out until December '99. So sure, the Pentium III can run all the same stuff, but that doesn't matter, because if you're interested to see what could be done in '98, then anything released after 1998 is irrelevant.
WinXP : PIII 1.4GHz, 512MB RAM, 73GB SCSI HDD, Matrox Parhelia, SB Audigy 2.
Win98se : K6-3+ 500MHz, 256MB RAM, 80GB HDD, Matrox Millennium G400 MAX, Voodoo 2, SW1000XG.
DOS6.22 : Intel DX4, 64MB RAM, 1.6GB HDD, Diamond Stealth64 DRAM, GUS 1MB, SB16.