What is the best metal to use for lead acid battery cables?
The metal is likely to be tinned copper. 
If the wires that the electrical shop (sic) gave you is satisfactory or not, is up to you, according to what current you expect to go in the wires, and what voltage drop you accept over the cable (and secondary, max temperature of the insulation material on the wires)

EDIT: According to new information;
If what you want to avoid is heat buildup and fire, what you need to do is choose heavier gauge wire. The heavier the gauge, the less heat is built up. If the wire is rated for 7.5A max, then that is what I suggest you stick to, unless you are comfortable with making the appropriate calculations and following criticality evaluation yourself. 


Anyway, the answer to your question regarding material is still: Copper - tinned or not. For more info on choosing the correct gauge, see Here and here