I want to buy a cheap time only lap timer.... I just need to buy the unit or I need something else... Transmiter?
You would need the lap timer and the transmitter. I have a Mini Lap Timer for sale, but not the transmitter.
That depends on when you plan to use it. Some trackday groups put out their own transmitter (that would work with an XT) and would not want another one out there to confuse others. STT for example puts a beacon out for all riders to use.