OT - kind of - Chevy 350 question

A buddy came over last night so we could weld some new floor panels and
replace rotten roll cage mounts on his CJ (see I am back on topic already).

When he went to start his 89 suburban it had power to the starter but the
engine would not turn over. We used a wrench to manually turn the crank a
little to prove that the engine , then by holding the starter it started to
turn the engine slowly, until suddenly it fired up. It acted like maybe it
was hydro locked and once we turned it a little to open the valves it
allowed the fluid to escape and turn over.

Then the exhaust seemed to have a little water, then we notice a mixture of
mostly coolant with some oil was leaking out from around the passenger side
/ rear area from the exhaust manifold. Also it was running rough, almost
like it was missing a cylinder. He commented earlier that he thought that
purchased some bad fuel and it was not running right. But now I am thinking
that this is the reason it was not running right.

Am I safe to take a guess that this is a problem with a bad head gasket?
How much is involved in replacing a head gasket on a 350?