You will have a very hard time finding anything that will match your requirements, as compromises will have to be made in different areas of the laptop. CPU wise, it should be fairly easy to find a laptop equipped with a Pentium III that will fit the bill, however finding a GPU that will "play more or less everything without major sacrifices" is very difficult. Nvidia didn't start having a foothold on laptops until they released mobile variants for their GeForce cards (GeForce2/4 Go etc) and 3Dfx cards were not on laptops at all. Your best bet is a variant of the S3 Savage4 or an ATi Rage 128 variant, both of which are slower than their desktop counterparts and not ideal in the first place thanks to driver issues which are further accentuated in laptop GPUs. Very often you will find Pentium II and III laptops that don't feature a 3D Accelerator at all.
As far as sound goes, that's another interesting piece of the puzzle as by this point everything had moved on to using the PCI bus. So, while Windows sound acceleration should be fine, under DOS it might prove to be a massive headache, as these sound cards tended to need a TSR or some sort of driver to get sound under DOS and while they usually did alright when it came to replicating the digital sound effects of the Sound Blaster, Adlib/OPL2/3 and/or MIDI support under DOS was usually pretty bad, often resorting to awful approximation of OPL2 through MIDI instruments (check out Sound Blaster Live! for an example of that). That could be a big problem.
And then you should also take into account other issues, such as that the battery for the laptop will probably be long dead and whether you find a replacement or not is not always up to you, plus the fact that laptop screens would produce a lot of ghosting making them unsuitable for games, granted, by the late 90's/early 00's the situation had improved immensely.
I think that what you're looking for is really really hard to find. There were quite a few mATX Pentium 3, Athlon XP and Pentium 4 motherboards, you could get a nice sleek and short case to house them and then they won't take that much space and you'll have a cool looking retro PC 😀.