Small tree

We have recently had an old laburnum tree cut down from a corner of our garden. We need a replacement tree to screen a shed and house. We are in our late sixties and can’t wait twenty years for a small tree to mature. The final height of the tree should be 15-20 feet. What variety do you suggest? Should we pay out for a semi-mature tree? We don’t know how far the roots of the laburnum spread.