1994 Chevy 3.1 Lumina Mini van head gasket

I have a 1994 Chevy lumina mini van with the 3.1 Last weekend I was coming home from NJ as soon as I was around Albany NY the van was over heating by the time I slowed it down as I was on the thruway and pulled her over the gage was in the red. I had her towed to a parts store where they were nice enough to let me change the water pump. I made it home about 75 miles she was running rough at idle but had plenty of power but was really eating up the gas. When home I checked the oil it was fine no water in it I checked the exhaust no smoke. I had a check engine light on. The codes were 14 coolant sensor/high temp and a 45 O2 sensor rich exhaust also the pcv was going nuts it wouldn't stay in the grommet. I took it to a service station my uncle swears by they called me and told me it was a head gasket. They gave me a price of $2,000.00 it was the rear head gasket. What I would like to know is I know it is tight back there since I have replaced the plugs and you have to feel for them. But could it be anything else like a manifold gasket only reason I ask is I came 75 miles with out it over heating power was fine until idle then she feels like she is going to stall. Also if it is the head gasket how hard is it to get to the back head is there any tricks to dong this. Someone told me that GM gives 12 hours to do it. The van has 115,000 on her. I just want to be sure it is the head gasket I can't afford 2,0000 so I'm thinking of doing it but if it could be anything else I would like to try that first. If not could you let me know how hard it is to do and any little tricks Ive done plenty of head gasket but never one on a mini van. I wish it was as easy as the ones on my 57 Chevy Bel-Air the room in that is like a garage compared to this mini van. Thank you!