how do we patch old plaster

Hi, we have a 100 year old home in Tassie and in two rooms we have large cracks in the walls and ceiling. The ceiling is the plaster of timber slats but the walls are a concrete type over bricks. We have dug out the cracks in the wall and my better half wants to use a render and then a plaster over the top but I think we should use something that has some flexability to it. As for the ceilings due them slightly bowing we do not want the expense of replacing them with plaster board can we just use a joining tape and then plaster over as this house is to be sold and if a developer buys it they will bulldoze it down. We are on 6 1/2 acres 8 minutes from the Hobart CBD so can anyone assist us with how to repair these problems without costing a fortune.
PS We did moniter the cracks over the past 4 years and they have not changed but the ceiling has due to sparkies in the roof.:)(


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