The power requirements aren't that high then, that's good. I was about to say you could easily adapt a computer's ATX PSU to give 12v and 5v at however many amps you need for the inverter (most good ATX supplies give more than 10A) but you also need 24v..
I haven't been able to read through that document, but when you say "DC Voltage Requirements", is that generated internally by the disk drive or do you need to provide that? You mentioned 1 DC input, not multiple.
I'd also highly recommend a pure sine inverter for this.
Amiga 4000: Apollo Turbo 040, 128MB Fast, 40gb HDD, CD-ROM (needs internet)
C64: 1541U-II, MixSID, WiFi, LumaFix64
ZX Spec, A1200, CD32, MacPlus/SE, A500, BBC Turbo Master (NASA), 2600, Acorn 4000, VIC-20, IBM 5155, GameBoy