when I first started looking for a macro, I found a classic 55mm macro 3.5 with extension tube which you can pick up for 50 dollars. your 3100 cannot meter, so you would have to take a swagger, and than looking at the historygram, adjust.
When I had my D80, I actually used an old external hand held light meter. Which works just fine. Lots of macro is pretty static, so,, a bit more setup is just part of the fun.
Once your budget moves up to AF newer versions, then really comes down to how close you want to be. I really like the results of the Nikkor 60mm, but got to say, I use the 105 macro
a bit more often.