There is minimum focus distance, but as far as how 'macro' this makes you depends on the range of the lens, the better way to measure a macro's performance with another lens is to find out the magnification. 1:1 is typical for a macro, meaning that the nut will appear on the sensor at its actual size dimensions, 4:1 means it will show up 4 times larger than it really is on the sensor. Mind you, this is as it refracts onto the focal plane, not the actual image itself. Nuts and bolts and the sorts could definitely benefit from the magnification - with a 4:1 magnification you could actually print a poster of a little tiny screw.

However, having said that, if the mentioned applications will be its exclusive use, particularly if you have no plans of making prints larger than letter size, then the EOS 5D and D300 would be very much overkill... If that is the case, get an entry level, a Sony A200, a Nikon D80, or a Rebel XTi or the sorts and spend almost 1/4 the price. But if you would like this camera to do a bit more, it might be worth considering every option.