Hi you have the top of the line Starmax then. They have the cache card accelerators. For hard drives you can run ATA drives and I think up to 120 GB as there may be a limit (not 100% as I have not used that particular Starmax) but I would guess so. Good news is those HD's are cheap, er well at least over here.....
This site has some handy info on the Tanzani II MB in your Starmaxi, albeit in German
http://www.macinfo.de/hardware/boards.html
and specific information for the 5000/5500
http://lowendmac.com/moto/5000.shtml
The Supermac S900 is the same as the Umax, it came in a lot of different versions. This has some good information here,
http://lowendmac.com/supermacs/s900.shtml
You might want to repaint the case instead, I would the wife painted a few compact Mac cases a few years ago for a newpaper story and they look pretty good. If your S900 has the twin turbo video card in it do not upgrade the CPU to the Sonnet 1ghz G3 as that video card will not work! I know because I have this card in a PowerTower Pro and the accelerator back in it's box because the video goes mad. OWC support tells me that I need a new video card, grrr but that is another story......
As for IBM drive support, you can always add a PCI ATA card. Was your card made for a Mac? If PC it may or may not work depending on the chipset on the card or just pick up a proper card off eBay granted I guess give the age of these clones I wouldn't go too wild upgrading....
Kevin