1991 Isuzu Pick Up 2.3 4cyl engine

I just purchased a 1991 Isuzu pick up truck with a 5 speed transmission and a 2.3 4cyl engine that the guy said had been sitting up for about the last 1or2 years. He did pour a little gas in the top of the engine to get it started and after the 3rd try it started and ran pretty good at an idle. The truck was pretty loud but, the guy pointed to the exhaust manifold on the side of the engine and said that it needs a donut in it to quiet the truck down because the rest of the exhaust from the manifold back was fairly new but, it did hesitate while test driving it around the block so I bought the truck because I thought that because it had been sitting up and needing a possible tune up and a oil change that would take care of that hesitating problem. It hesitated and stalled every time I slowed down or came to a complete stop but, it did start back up every time and when I got home about 10 minutes away from where I bought it from I saw and smelt something burning and it was the catylitic converter on fire burning [bright orange]so bad that I used a water hose to put it out. The truck ran okay but what do you think is the problem and what can I do to repair it. Thank you in advance for your response.