Make sure you get a good macro lens, a real macro lens that does 1:1 reproduction ratio and not some cheap sigma zoom job that says macro but only does 1:3 etc.
All macro lenses regardless of focal length will make the subject the same size on the sensor, and thus the same size in the frame. The difference is purely in terms of the minimum focus distance, which is the distance form the sensor to the subject to achieve 1:1. Is important to realize the MFD is form the sensor, not the fornt of the lens. A 50mm MFD with a 50mm Macro lens may only give you about 50mm of working space from the front element. So it depends what you want to shoot, small insects and the like that get scared then you will want a longer focal length lens. Nikon 2000mm /f4.0 Micro is golden for this type of work. Conversely, if it flowers or still life then shorter lenses are easier to use
I think you could do better than the D80 though, newer cameras have a much better sensor. You don't need a fancy camera, macro work is usually done with manual focus.
You should really budget for a tripod, ring flash and focusing rails to get the best results. When working at macro reproduction ratios the effective aperture becomes quite small, and you have to close the lens right down anyway to get anything in focus. Thus you have very little light and a tripod becomes necessary. Dof is only going to be a mm or 2, so the slightest movement will send the subject out of focus, so focusing rails helps you nail the focus. ore over if you want more the subject in focus than a thin slice you will need to take multiple exposures and stack them together in software, again you will nee focusing rails and just move the camera forwards by 0.5mm at a time. This also helps prevent the need to stop the lens right down which causes diffraction.
Look for all that stuff second hand. It wont be super cheap but will hold its values so if you get bored of the hobby you can liekly re-sale with little to no loss.