right to start that abit confusing (well to me anyway)

first off you need to check weather its the cable or caipler or drum since i dont know which you have if you have rear caiplers its easy to check where the problem is just jack up the rear of the car on the side thats failing and get under from the rear of the car and look at the back of the caipler and you will see where the cable goes to the caipler its on a little metal arm, what you need to do is get a hammer or some thing big and hard and spin the bad wheel then use the hard object to push that arm on the caipler towards the front of the car if it stops the wheel then its the cable thats at folt if it doest stop the wheel then its the caipler at folt.

if you think you can change these items on your own but dont know 100% on you car i can tell you if you want there not to hard to change.

as for your other questions
no discs and drums anit the say and thay use diffrent hand brake cables.

no all cars use diffrent lengh hand brake cables so you need to get the correct one for your car.

if you have replaced the discs did you reset the ajuster on the caiplers when refitting? as if you didnt do it it would cause liitle to no grip on one caipler..
it takes 2 mins to do and you can do it without jacking up the car using the way i said above just push the arm forward on the caiplers a few times with the hand brake up and thay will stop about half way and will become hard.

any local halfauds sell them but best go to your local motor factors as thay will sell the same thing but alot cheaper.