One of my bikes has Shimano roller brakes front and rear, and another has an earlier Shimano roller at the back and a disc in front.
I have little faith in discs and have never had a satisfactory disc installation; they all ate pads at an enormous rate because I'm heavy and my gear is heavy, and I have a heavy hand on the brakes because I like speeding downhill and at the bottom of my hills one often finds a T-junction.
The later Shimano roller brakes are in some senses a superior solution to disc brakes, but they require a torque reaction arm on precisely the same pattern as disc brakes, with which they're largely interchangeable. Scroll down on this page --
http://coolmainpress.com/BICYCLINGsmover.html -- and you'll come to some large photographs illustrating the torque arm mounting and the huge cooling discs. The big black hubs you see are not part of the brake but a hub dynamo and a hub gearbox. The roller brakes mount to the side of these.
Early roller brakes from Shimano were limp but the later ones -- -75 series on my Trek Di2 in the photos linked above and newer as in higher numbers -- are ferocious stoppers, and don't complain about being used as drag brakes on long hills when I want to slow to talk with pedal pals who ride their brakes down every hill. Between me and my painting gear, there's often a minimum of 130kg on my bike, so the later Shimano rollers cope well.
However, I agree with Martin: if you want discs or rollers, you'll need a new fork.
One minor irritation with the Shimano roller brakes is that the closure for the service port (a complete service consists of squirting a shot of grease through the port) is a miserably little rubber plug, untethered, easily lost. Buy several spares when you buy the rollers, if you decide on them, and a few of the tubes of special grease. If SJS doesn't have them, try Petra Cycles, who specialize in rare and wonderful Shimano parts.
***
It may interest you that both the bikes with rollers are in the loft, three storeys up, from where they won't return to the road in a hurry; they're simply more brake than I require.
Instead, I've found the best brakes for my use -- heavy cyclist and heavy gear -- is Magura's rim hydraulics. They cost about the same as cheap discs, they're sealed for life, zero service except brake blocks, they work very progressively, they clamp like crazy in an emergency, they're very economical of brake blocks (10,000km is possible in tarmac countryside use with zero commuting), and they have a huge cooling rim to work on. The two suitable lines of Magura rim hydraulics are labelled -11 and -33; there used to be a difference in clamping power between them, so I chose the lower clamping force -11 series as more progressive and threw off the fork bracing plate too for the same reason, but today both clamp the same and the more expensive -33 offers trivially better convenience in adjustment for the substantial difference in price. Unfortunately these wonderful brakes also require specific socketry to bolt the calipers to, but at least up near the fork shoulders, not down on the slender part of the fork.
***
I seem to remember a recent thread here on the Aral drum brake's availability, if you want to search for it.
