The 400D is digital. You cannot put digital-series (EF-S) lenses on film bodies — don't try it for want of the damage this can cause.
Maybe consider something like a Canon T90 or Olympus OM4 or Nikon FE2 (can be had for around AUD$220.00) and adapt for your Tamron? I am not sure an EOS 3 body could be had for AUD$300, more likely in the AUD$500 range. It now would seem to me that your price range limits will dictate what body you pick up, unlikely the 1V, only a little less likely the 3. A few others choices: EOS 50/50E (ECF), 5 (flawed ECF), 30/33, just good basic cameras (my first and most successful B&W triptych shot 14 years ago came through the EOS 50E and a humble 20mm lens: the printer did the rest!).
Note that true macro (1:1) photography will not benefit from, nor be facilitated by, autofocus. "Close focus" (<<1:1) might be OK.