wut should i upgrade yokomo drift master first?

i am planing to buy a yokomo drift master, futaba 3pm-fs pro radio system and mamba max 5400kv brushless motor and esc. also, i want to use about $HKD1000 to upgarde. what should i upgade first? i want to drift it, wont race too much. so top end speed is not my cup of tea. welcome any suggestions. also, i have questions about wut is "珠差ball differential??) / 頭單(one way diffierential??) / 直濕(???)"?? thanks

what is "中單" also???