Is the screen pic asking for Kickstart or Workbench? If Kickstart, try re-seating the ROM chip, replace if it is bad. If it is asking for the Workbench disk, then it might be a bad floppy drive or CIA chip.
A1200 HD depends on which OS is installed. 3.0 and 3.1 without any patches, you should stay under 4gig with max. 2gig partition size. 3.5 and 3.9 can use larger.
Also, welcome to A.org
