95 Cadillac Seville SLS w/northstar engine----read-out indicated overheating w/ temps reaching 123 and then engine cut out.---car had sat for approx. 9 mo.---added seafoam to help stabilize fuel---now won't start at all---turns over but doesn't fire--has several codes listed-P0022,P0024,P0030, P0039,P0070,P0073, P0075. Fluid levels are appropriate and freon for air conditioner is full. Where do I start looking for the solution? Cadillac dealer seems to want an open end $$$$ to even look past basic scan

Optional Information:
1995 Cadillac Seville SLS 32 valve Norths

Already Tried:
dealer scan for possible problem source