Mechanics - replacing head gasket vs the engine...?

If the head gasket failed because of overheating you might consider another engine as overheating can ruin piston rings and cylinder walls. This can lead to the engine being an oil burner once repaired and a compression test, wet and dry, should verify this.
Coolant diluted engine oil can also wipe out crankshaft bearings, cam lobes, etc so the engine oil should be inspected for this problem too.