It was me who adviced you to register here, so welcome. As I said in reddit, you will most likely get more food for your thought here than in reddit, especially when all posts get buried very soon there and comments dry out. Also, reddit answers were in many cases IMO wrong for what you are mainly seeking starting from suggestions of socket 7/pentium MMX systems.
I said in reddit that you need to decide a focus and era for your system as the 90s and early 2000s were a time of rapid change and it is extremely difficult to build a system that can perform well spanning 10 years or so. Early systems are too slow for the late period, late systems are too fast and pita or impossible to slow down enough and you lose ISA in early 2000s, which does mean bad DOS compatibility.
Many DOS games were released up till 1997 until games overwhelmingly moved to windows. You can in many cases run these late DOS games quite fine on faster machines, but the deeper you go to DOS era, the more problems you will encounter. If you accept that for this first win98 box your target is late 90s windows games, it will be easier for people to guide you and for you to actually start sourcing parts. And if your expectations are realistic, you won’t get disapponted if your system isn’t performing well outside of the era your priority is.
I also said in reddit that you need to think about glide support, ie Voodoo accelerators, because they were the accelerator for quite a while and there are games that don’t support other than that or the D3D renderer looks worse. If you want to opt one, it will be somewhat costly nowadays, but in the end it it is up to you and it is not mandatory. Me personally would prefer putting the money towards Voodoo instead MT-32, because that is something that is more or less only useful for earlier DOS games as people here have already commented.
IMO P3 600 is a good option for what you are aiming for. And with ISA soundcard you can definitely play many DOS games. Someone here also pointed out that for AGP graphics cards, nVidia is a good option because of good DOS compatibility and this is true. A good budget option could be GF4mx440/460 for example. They offer slightly better performance that top end GF2 models, better image quality and are often cheaper than top end GF2s. It is pretty overkill for 600MHz P3, but you won’t need faster one and there is no harm having such card in the system.
I suggest that you start browsing parts (or systems) from ebay or equivalent which you are planning to buy and then post it here so people can comment if you are heading to right direction. Many users here have huge experience from diifferent systems, motherboards and graphics cards. There are for example motherboards that are hoplelessky buggy, slow or otherwise something that should be avoided. By posting your components list here you most likely will get feedback if you are possibly making a bad purchase.
This is a great hobby, have fun!