I would argue that the gas tank is a bigger problem than you're making it out to be. Imagine a rock or piece of debris bouncing along the road and hitting the right spot on your gas tank. You could be engulfed in flames within 2 minutes. In an older car, I had a rock bounce under the car and punch a hole in the muffler, which is thinner than a gas tank but indicative that it could happen.
I'm of course not discounting the other issues. They should both be addressed if you keep the car. Ideally, you could fix all three (muffler is a little more difficult - you might have trouble getting the old muffler off) but if you must take it to a mechanic, you should consider buying the parts beforehand and having them install it. Most, if not all, mechanics will charge a premium on replacement parts as a way of keeping profit margins up.
Ultimately, if you're going to sell the car, I would recommend selling "as-is". You would not see your repair investment recovered if you were to fix it and then sell it.