My initial suggestion would be to go for the 105mm Micro because it could double as a portrait lens.
I've seen reference to a lens-reversing ring that allows a standard lens to be used at high magnification. I think that Nikon makes one of these. It might be a bit less expensive than buying a new lens.
You may also want to look into (ha! extension tubes or a Canon 500D close-up filter. Either of those would probably be much less expensive than a new lens.