John, here is MO as I've Been There, Done That deal....
* Should i convert to discs?I have to change the coupler anyway,as it's shot.
yes, Get the Kodiac Stainless Steel disc kit.(You might get away with the 'Silver Cadmium' which is cheaper) but I would go all Stainless. It includes the Calipers, Hydraulic Lines, Rotors, Brackets & Instructions
* Is their a big difference in stopping power between drums and discs? Yes, The drum will work the first or second time out and then once it gets hot/used and then wet, it will never stop rusting. Bottom Line - They just will not last in the H2O environment, regardless if it Fresh or Salt water (Salt Worse of coarse). They don't get the daily use like your vehicle, therefore the stopping power decreases continuously.Those Auto wash connections/gimmicks don't work either on the steel drum brakes. Steel + Water = Rust = Big Problems.
* Is it a straight foward job to change,and are ther any common problems to look out for when doing it?.
yes, all instructions on "How Too" are included. It will take two people to properly "Bleed" the system and get all the air out of the lines. Be sure to have plenty of extra brake fluid. Get a small bucket too catch the fluid so you can pour it back into the Actuator. Also, You will have to get a "5 prong" wire set up on your tow vehicle & trailer for the back-up solenoid to operate. Make sure you position the mounting bracket at the correct "clock position" as instructed.
BTW: You can Goggle this kit and I think there is a "How To" Video out there
Here is Link to a discussion from Continous Wave---->
Brake Discussion
Good Luck, Capt_Chuck