There really isn’t much option, if you don’t spend the money to repair it you’re left with a car that is worthless and you are faced with spending more to buy another car. As you say it is in good condition otherwise I would recommend you went ahead and repaired it. If it does have a blown head gasket, as you have been told, there is no other way of repairing other than replacing the head gasket, and there’s no way of reducing the cost of the job. To minimize the chances of being ripped off use a reputable mechanic, perhaps one who is well known and respected in your local area, take a friend or relative with a knowledge of mechanics with you when talking to the mechanic, and get two or three quotes for the job.