The above guide was helpful, but what I did was go to free race, solo, and then sewer surfin’. Once the race starts, go to after the second pylon and there are two rows of coins with six apiece. Once you grab them, after a few seconds they reappear. I just stayed in this area and went back forth collecting them. I personally needed about 900 coins to finish buying everything. It took me about 24 minutes to get the coins and finish the race. You must finish the race for the coins to count.