Your first pair of sentences is a very interesting one. It shows how complex English tenses can be. To see the difference, you will have to use your imagination. (I'll change the sentences a little to make it easier.)
Imagine that yesterday you saw a woman standing in a field. You saw a man move quickly in her direction. Today, you can say, "I saw a man running towards her." This means you saw him in the act of running towards her. He was doing it right then. Now imagine that, before the man reaches the woman, he stops, turns around, and runs away from her. Now you can say, "I saw a man run towards her, stop, and run away." This means that you saw him complete the act of running towards her. When you talk about the past, "running" means it was happening right then and you did not see it finish. "Run" means it was happening right then and you did see it finish.
Your second pair is a little easier. (Thank goodness!) "If I had enough money, I'd buy a new car" means "I want a new car, but I do not have enough money to buy one. If I were to somehow get money, I would use it to buy a new car."
"If I have enough money, I'll buy a new car" means "I want a new car, and I have some money. There is a chance that I have enough money to buy a new car. If I do have enough money, I will use it to buy a new car."
The difference between these two sentences is probability. The first sentence talks about a hypothetical situation that is impossible, at least right now. "If I had enough money..." means "I don't have enough money, but if that were not true..." The second sentence talks about a hypothetical situation that is possible right now. "If I have enough money..." means "There is a good chance I have enough money, and if I do then..."
All of your sentences have very subtly different meanings that are difficult to explain but can be very important. If you need further help, please post again.