I "retro fitted" one to my 850 (2000) and all the sensors are already there.
Check to see if there is an 8 pin i think connector at the front of the tank ready and waiting.
Had to buy a new plastic surround which was about 18 quid to make the job look good, as my plastic cutting skills left a little to be desired on the original
I bought the unit s/h from Sherlocks
Doesn't make it go any faster though!!!