I think the best way you can judge a mechanic in a situation like this is whether they mention specific information. Here are some questions I would have:
How did he determine this? Which gasket? The head gasket?
Which assembly? The cylinder head? Why?
Did you inspect the water pump yourself? Where are there no bubbles? Where are the oil and water not mixing?
If the car's power is fine, then why do you need a head gasket and cylinder head change? Because it overheats?
Visit @rajaatta or @dani_kk.