Valve cover leak or head gasket?

Hello I wonder if someone can help. I bought an 05 Ford KA on Saturday. The seller told me it was overheating and needed the thermostat changing. I drove it home (approx 2 miles) it was badly over heating. I left it parked up until Tuesday when I got a mobile mechanic to replace thermostat. He replaced it charged me £90 then told me the head gasket was gone and I may as well scrap the car. I have since seen on the internet that it may only be the valve cover but how would I know? I really don't have the money to take it to another garage to find out. I have read that there maybe stuff in the oil, but the oil has recently been changed and so there is nothing in there, the mechanic gave his reasons as the fan was cutting in too quickly when the engine was running but stationery, (although he was revving it high and for long periods) also that the pipes were getting too hot too quickly. Any advice would be greatly appreciated. Thank you.

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