i didn't buy any extras but i will be buying the boost sensor and egt sensor & harnesses so i can have complete datalogging
i got the bundle that came with a screen
it wires directy into the ecu for datalogging of tps rpm etc and for powering up
it has it's own harness for the 02 sensor
it doesn't improve a vehicle at all
it helps you make sure you don't blow it up
by having a screen there that tells you your exact air fuel, it's an extreme comfort thing