As far as wide angle lenses go, the Tokina 11-16mm f/2.8 gets some very good reviews
Have a look at www.graysofwestminster.co.uk , they have the 70 - 300 AF ED D lenses (f4 - 5.6) from about £135 second hand and guaranteed for 12 months. I recently bought one from them and it was good. You would have to pay more than double that if you wanted a VR lens though.
By the way, I got the recommendation for that company on here and I certainly had no problems dealing with them.
Ernie
I can recommend the 60mm AFD Nikon Marco, really good sharp lens and a good price 2nd hand. You don't say what sort of macro you are doing, but its a great general purpose lens (working distance a bit short for insects).
Kaolin
I think you should consider how the 18-105 limits you more than anything. Lack of sharpness? Lack of reach beyond 105mm? Macro is easier, if you are serious about it then a Nikon Micro-Nikkor is the way to go. As for landscapes, how is the 18-105 limiting you in that respect?
freecom2