Firewarrios all the way?
First advice: TAU CODEX
Now, i`m in a big hurry so i`ll jsut give general advices, Tau Firewarriors really are the backbone of your army until you get to 1000+ points armies
For 500p all he needs is a Commander (someone else will have to detail weaponry), 2 full suqads of fireawarriors (with bonding knives since they don`t cost that much) and kroot for all the points left
Kroot are there as a fire sponge while the commander + Fire Warriors fire away
I`ll try to come back with more details