You can get a cam belt kit (and a whole lot of other coop parts) from
alternative autos but changing the cam belt is not straight forward, so your mechanic would need to be familiar with the coop to give you a price - or he'll be in for a shock when he comes to do it.
£450.00 for the belt change at a specialist is more then reasonable. and will include all tensioners, pulleys, water pump and auxilliary belts

Get the seller to knock off a couple of hundred or have the belts done before the sale