400Mhz should work just fine.
As for the hard drive limitations, I don't think a system that old would have 48-bit LBA support in the BIOS. You can either buy a separate card that allows for 48-bit LBA support or have support in the operating system via DDO. Due to the way BIOS's load drivers, a card would be the best option. The worst would be using a DDO program since you lose almost any chance of moving that drive to another system later.
Just remember you can always partition smaller and expand it later...
Cheers!