I have tried one of those SATA to SDCards as a cheap SSD like alternative. Problems I ran into is that it only worked for Linux and Microsoft Windows refused to install to it.
I'd just pick up a cheap used IDE HDD if you need one and dont have one. For Win 98 a small HDD would probably do such as a 8GB. It all depends on what you will be running for games.
Also to mention socket 370 CPUs are cheap. You might be able to upgrade it to the better Pentium 3 for around $10 off of amazon or ebay. It would also open up your game selection for older games that may have been too demanding for a celeron, but Pentium 3 was perfect for them without lag. The 66Mhz FSB Celerons I try to avoid and go with a 100Mhz or 133Mhz FSB Pentium 3 if the motherboard supports it.
Ive looked on Ebay for a few, but sadly whatever is cheap to me hits me again in shipping. For what ill be doing i know 500MHz Celeron is just fine. I was able to play most of my games using a beat to heck IBM A20m thinkpad that featured a 500Mhz Celeron, 128mb Ram, 12Gb hard drive and an AGP 2x - ATI RAGE Mobility M - 4 MB SDRAM.
I'd still be using the IBM but it was given to me because the right side is completely missing not only the plastic that kept the hinge together, but part of the plastic surrounding the rear. From what i was told, it was thrown from the bed where the user left it.
The games i plan to play on the machine are as follows: GTA 2, the DOOM series, Jazz Jackrabbit, Titanic: Adventure of out Time, Re-Volt, prince of persia (original dos) and Dyna Blaster as well as some later Microsoft games such as Monster Truck madness, Mid-town madness 2, NFS porshe 2000, Toca tourning cars 2 and a few other titles im sure im missing...
Also ive seen many youtubers like Druaga1 install windows on CF cards.. the 8-bit guy did too. i might stick to a 40GB IDE then or maybe get a IDE TO SATA.