I assume the outbuildings have pitched roofs? You could use a material called Colorbond, which is a powder coated corrugated steel cladding, and makes a good, long lasting roofing material. It's extremely popular in Australia, where they clad buildings in it, build fences etc. Another idea to consider is to use reclaimed tiles or slate, much cheaper than new material.
Bear in mind that you will need a licensed specialist contractor to remove & dispose of the asbestos, and they will not be cheap.
Failing that, why not demolish the outbuildings and sell the bricks/stone etc to a reclaimed materials yard, landscape the area where they stood, and bingo-150sq metres of extra garden.