Mechanic & Car Service Job - White River, MP, 1240

I bought my Subaru Forester
2.5XA 2007 in December. It went well, until it started heating up and oil was mixing with water. The concensus was that either the head gasket had to be changed or the block itself. It was the head gasket and when the motor was reassembled, it wouldn't start. We got it started with another battery and drove into town, where we bought a new battery. On the way home at a robot, the revs went below 1000 and stalled. We were unable to start the engine again. We got towed home and were asked if we had changed the sparkplugs, which we hadn't as they get the four pistons to start. We put the sparkplugs and the next morning, despite the plugs and new battery, it still wouldn't start. I am hoping that a professional mechanic could give the motor a look, and see ifbthe plugs were put in the right order. If a more severe problem is discovered, then I would have to sell the car as a non runner. This is a shame as the car is in good condition, aurcon, wipers, digital radio and GPS all function. Thank you for your advice/diagnostic.