The power supply is what you need to worry about for -5v.

Just use a newer ATX power supply and an adapter to add the -5v.

Look up seller antronst on eBay.
https://www.ebay.com/usr/antronst

If you need more than 1 ISA slot you need a BX motherboard. 8gb of RAM? No way. The maximum you can get on a Pentium III is 4gb on server motherboards, which are rare and expensive. For SATA, no onboard solution, you will need a PCI controller. I recommend the Promise SATA300 TX2 or TX4 if you want to install Windows 2000/XP or the SATA150 TX2 or TX4 if you are going to use Windows 98. Onboard USB is 1.1 only, you'll need a USB 2.0 card. Use one with a NEC chipset, won't give you troubles. Most server motherboards don't have AGP too. So in the end you better get a BX motherboard as it has more than 1 ISA slot and has AGP. For SATA and USB 2.0 you can just use PCI controllers. For RAM, BX supports up to 1GB but you will need a motherboard with four slots instead of three.
There are some VIA Apollo Pro 133A motherboards with more than one ISA slot, such as the QDI Advance 10 though. You get the advantage of getting 1.5GB of RAM though and 133MHz of FSB.