"The 'expert' at PC World told me that if I buy a 400W PSU it will fry my MB but I don't understand this. "
He's talking out of his hat.... You're right the components draw what they need.
"What are the chances of damage to any of the components in the computer, visibly everything looks fine.. "
You won't know until you try. Seen it leave everything intact, seen it blow everything except the case. Chances are most things will be OK though.