Anyone Help!?

I’ve recently purchased a Audi A3 S Line 2013. I brought the car and everything about the car was mechanically sound. Did all the usual checks coolant oil ect the only issue it had was the heaters were blowing cold air when it was on hot. Spoke to a mechanic and he said it would more likely be thermostat or something. Took it for a test drive and it drove spot on had plenty of power and drove really well, I drove it home for 3 hours no issues at all. I’ve taken it into a garage to be serviced and while checking the coolant the bloke smelt it and said it stunk of exhaust fumes, after a couple of opinions they suspect the head gasket is on it way out or a cracked head or even EGR cooling system which may explain the heaters not working?? Any help?? Thanks