If there are no drivers ready for Win98, then you can not get it done. You might want to go for a bit older hardware.
Here I am talking about P4 Socket 478 stuff or Socket 754 with something as simple as a Sempron.
P4 might be faster in benchmarks, though eighter way, you will have a killer Win98 rig.

For the GFX, then go for something AGP type of card. That can be a Geforce up to and including GF4.
Just keep away from MX cards. Unless it is a GF4-MX460 as they are ok. Just avoid GF2-MX cards.
If you can source a cheap Radeon-9600, then it is a good choice as well. It has some quirks when you
want to play them really old Win98 games. No perfect pc is out there anyway.

Sound... Hmmm.... If you want to go the cheap way, then something like SB-Live should be good as
a starter card. Shure better alternatives is out there, though if you want to go cheap, then it will do.

Again... Remember to check for Win98 drivers before buying. And sometimes this era, can be found
cheap locally. You know... When old people discard stuff and families have some old retired system
in their basement or anywere else. Hope this helps you find a good and fast Win98 machine.

The hard part is, people don't ever seem to advertise their old computer stuff. I got a 33MHz 486 and a 6X MHz Pentium system, each in identical steel tower cases with plastic bases, plus printers and a monitor, with all the fixin's, for $20 at a yard sale a couple months ago that I stopped at on a whim. When people do advertise having computers, it's usually junk from the early-to-mid 2000s that is too new to do classic computing stuff on, and too old to do anything like simply browse the web. The last time I got any other useful computer stuff at a yardsale or anything similar, was a few Macs and semi-rare pre-Mac Apple systems at a church rummage sale about twelve years ago, which I no longer have.

Never trust your family to safeguard anything for you.

Try your local computer recycling centers or Goodwill. They usually save the newer parts 2010 to 2017 and anything older than core i3 computers they will trash for scrap. I know because if I didn't save some parts at work they would be melted for the metal recovery (silver, gold, platinum). I have gone to other recycle centers and had complete computers given away just to get them out of their way because "no one wants to buy a Win 95 or Win 98 PC anymore" so just take it home. You might not always get them for free but some places charge very little for complete computers that are over 6 or 7 years old since no one asks for them anymore.

They are still out there, earlier this year I got a working Slot A/Voodoo 3 PC and a Mac G3 missing the PSU out of the same skip. But 99% of the time it will be later stuff for sure.
As Brostenen said P4's especially Socket 478 can run Win98 almost trouble free and are still dirt cheap.

If you get a system with AGP,PCI and IDE it'll be smooth sailing, some later hardware can be made to work but not out of the box