The light could have just come on because of the hose but I wouldn't get too comfortable with that from the sounds of it. If I were you, I would have the guy you bought it from replace the gas tank, and tell him if he does not that you will contact THE BETTER BUSINESS BUREAU and let them know what's up. Don't let him jerk you around and keep your foot down. Be as level-headed about it as you can and don't make personal threats, just let him know you mean business and that if he doesn't take care of this you will contact the necessary people who can hurt his business.
Second thing...I would have a trusted mechanic look over the whole truck and it's systems for you. To find a local shop that will suit your needs as far as cost and loyalty to the customer, go to
Car Talk, they have a data base that pulls up shops all over the US and customers rate them. Get documentation of any real damage on the truck that could have been taken care of by the guy you bought the truck from. Obvious damage like a cracked gas tank is something that could have easily been spotted.
As for the cost of a new valve body...I'll talk to one of the guys over on the local forum cause I think he replaced his himself. So I'll get back to you on that one. But yes, the Ranger is pretty much the only vehicle from Ford that is worth anything...it's lived up to it's name through the years.