A gallon should be plenty. If you are brush painting it should be more than enough and you should have some left over if you are spraying. If you are doing different colors, I might consider getting quarts especially if you are brush painting.
They do make "metal" based plastic body fillers but I do think regular bondo should suffice. Do not take my word though, read the can.
Since this is your first time painting I do think it's still a good idea to practice some, maybe on a high wear area like the floor. It's a good idea on the floor to use a light to neutral color. Light gray would be good. White is cool in the sun but may reflect too much sun, giving sun burns or for lack of a better term, "snow blindness". Black is too dang hot. A neutral, gray, green, blue would be about right. You want a color for the inside that will not burn or blind you.,