I am thinking of buying this 2004 Mazda 6.

Hi, I am thinking of buying this 2004 Mazda 6 sedan that this idiot allowed to overheat thus from what he says - he blew the head gasket. He only made short runs in town with it in this condition - for how long I don't know. I am sure the car finally gave up which at first I was thinking (Besides the head gasket needing to be replaced) that it could be one of several things - 1. Hydraulic Lock in the motor due to coolant filling a cylinder - 2. Starter was fried when he most likely tried to start it again until he killed it - 3. The battery despite him saying it has full power is only able to turn on the dash lights and deliver nothing to the starter as it could be very well that a cable end is not connecting correctly to the battery post thus not completing the circuit - The only other explanation I can come up with as I am not for sure about his key or keyless entry device is that the computer may have put itself into safe mode.

I am savy enough to do the work myself and this is how I make my living these days buying and selling cars - I would hate to pass on this 04 because is a steal at this price $$ providing it is something simple enough (besides the head gasket) that I can handle to get it home and strip the heads off - but if the motor is fried then I don't want the headache.
Can someone that has previous experience with these 04 Mazda 6 sedans give me the top four or five things to check first and in order please? I would feel a lot better if I had a better understanding of this - Thank you in advance from Hawaii !
1.
2.
3.