Good 1st question & to be honest I'm not sure of the answer
Maybe one of you more epxoerienced members can share their thoughts on that one.
As for choice of lens, then the likes of Sigma will come in cheaper than Nikon, so take a look at the
Sigma 50mm Macro which can be had for about £160, followed by the
Sigma 105mm Macro at about £240.
Alternatively if you already have a
Nikon 50mm f/1.8 lens you could consider extension tubes. I know the likes of Steve have achieved good results with this combo.