if this is your reason for wanting rear discs....rear disc conversion is just wasting money. even with rear discs ur brakes will still overheat coz primary braking is done at the front and the front pads are the ones that are overheating..so instead of concentrating on the rear.. concentrate on the front instead....the rear is more of "additional support" with clamping force much lesser than the front. which is why u see the rears are always smaller than the front.

if this is ur worry, get higher temp brake pads instead...even bendix metal king will be more than sufficient for genting... be it normal drive... or spirited driving. optional would be steel braided lines as they will help improve pedal feel and maybe even dot4 racing brake fluid like the Motul RBF660 or EBC BF307+ to ensure the fluid can take more heat and doesnt over heat.... but to be honest... just the brake pad alone will give a significant upgrade already. i wouldnt even bother upgrading to a bigger disc brake setup.

if anything, along with better brake pads, better tires should be on ur list instead.

for your fishtailing comment...when u brake, weight of the car is transferred to the front (ie u see the car nose dive). this means the rear will have less weight putting pressure on the wheels which means less traction on the rear tires. which means the rear is easier to lock up. u do not want any of ur tires to lock up as a lock up means loss of traction. so if ur rear brakes bites hard when u have minimal traction at the rear, this will lock up easily... and lock up... more fishtailing, spinning, round n round etc.

for drums + wet conditions... actually.... i think the drums would do better in the wet than discs mainly coz the drums are enclosed and water will not interfere with the friction surface/ contact area whereas the discs will have water coming in between the pad and disc contact area. plus, the water will actually help to cool the entire drum brakes down. my own experience with my old bike (yes, bike, not car, but same concept applies) which had drum rear and disc front...the drums will actually improve and performed sooo much better than the front when i ride in the rain whereas the front bite is significantly lesser (sometimes almost non existent).

so... my advise.. FRONT brake pad + tires.