Have you figured out how small will your subject be. If it is very small - less than 10mm macro is needed. If it is more than 25mm then any long lense will do. Again if it is skittish insects you need a long lense, 105mm to 200mm, to maintain the distance, for flowers on other hand a shorter 60mm will be better.
One thing that I have experienced is that at close distances, the DOF is very low, so you need high F stops or focus stack when ever possible. At high F stops diffraction comes into play, as does CA (which actually gives fringes to edges, so diffraction effect is visible at relatively lower F stops). So you have to search for the optimal focal length and a low CA combination.
The on-board flash of my D3300 works well enough for close ups with normal lenses, but with the 105mm AIS, when the aperture needs to be around F32 or more, the on-board flash gives up. In such case an external flash is required. To calculate the max power required :
D = G/F
where :
G is the guide number.
F is the aperture (the number - 11, 16, 22, 32, 45 etc).
D is the distance, in the same units as the guide number.
So if G = 100 feet and F=32, you get D=3 feet.
Now D has to be more than the distance of the object being photographed, and it varies between 1 and 3 feet depending on the macro and the magnification used.
NOTE
. While calculating the maximum flash power required, keep at least 30% as a reserve.
. The guide number in many models increases as the coverage cone decreases at longer focal length lenses, so take that into account.
