After conversion, i bought my lens VC version at around $600+. I remember there is a thread here about which shop in HK is more reputable to buy from. =}
hmm...maybe u might want to save a little more and get a tokina 11 - 16 later, since you mention you are at 18mm most of the time... the 11-16mm with 2.8 might give ya a even wider angle...and you might end up at 11mm after most of the time
if you're at 18mm most of the time, getting the tammy 17-50 satisfies that plus you'll get f/2.8 aperture. do you use the 105mm end of the 18-105? if not, then go with the tammy