Hi, from what you wrote, you have three choices of display namely (i) amiga video 23pin (ii) amber VGA (iii) VGA from GVP card (is it IV24?). You sure you've tried all?
The transformer should be no problem (it should be 250V->125V not 250W->125W), since you heard the hdd spinning up.
Also, on the mobo there's a jumper for selecting PAL/ NTSC. Have you tried that?
The last but unfortunate possibility is that the dishonest seller had ripped you off by selling you an a3000 with faulty display ... :-(