If I recall correctly from memory (I don't have my handbook with me), BRCA minimum 4WD weight is 1588g.
As for additional weights - stick on alloy wheel weights are what most people use - widely available in 5g and 10g 'strips'.
They used to be lead but now they are more environmentally friendly - so they're a bit bigger than lead to package.
Brass is also a useful material for adding weight as it is quite dense.
As for where - difficult one to answer without going into weight distribution etc etc. But lower rather than higher and make sure you retain a left/right balance once fully loaded.
Front / Rear balance - you use to fine tune the handling.
Big can of worms though - and hugely topical right now
HTH