So the system I am eying has the following: MCA/PCI based motherboard, 3.5" 1.44 Mb FDD, SCSI CD-ROM, 2 SCSI HDD, ethernet card, modem card, MCA WiFi card (without the dongle), PCI Fast-SCSI-2 adapter, IBM sound card, all untested in a case with no apparent rust or other dirt. Shipping is likely to be kinda high (maybe 200 bucks), but is offset by a very low price of the system itself. Worth it?

It's most likely the PC Server 520 - with 2 PCI slots and 6 32 bit MCA slots. It's a 2 CPU design. It's not worth $200 shipping. It's a standard 430NX motherboard with a PCI-MCA bridge, a Cirrus Logic GL5430 intergrated graphics and an Adaptec AIC-7870 SCSI controller. The WiFi card is most likely some proprietary early wireless product.

If it's the PC Server 720, then it's more interesting, because it can take 4 Pentiums and it must have some custom chipset (by Corollary), but the MCA slots are most likely still just bridged off the PCI bus.

Microchannel sound cards on their own are uncommon so I'm ballparking the market value (IE: not what I'd pay for one) is $150.
Pentium MCA machines WITH PCI also indicate it is one of the last MCA machines IBM made. Regardless it sounds like it's worth the gamble.