Here is a link to SIS 55xx model chipsets and BIOS types.. plus mobo providers..
http://www.wimsbios.com/chipset/2A5II.jsp
Probably no success in finding a newer BIOS than what is had. The BIOS has to be tailored to the mobo design, a different one likely won't work.
And as to hard drives, getting a really new drive of less than 100 GB is not easy. You may find some laptop hard drives of less than 100 GB, and they should work with the system if an adapter to standard IDE is used. Set the primary partition to no more than 8 GB, and create an extended partition. In that, you could go around 10 GB each additional drive partition, and it should work. But with only 256 MB system memory, you could not use newer than Win XP as an OS. A Linux system would be more compatible.
In checking my archives, I found nothing newer than what you have for a BIOS.