1/2) "Street-legal" is used to refer to cars that you can legally drive on public roads, which typically have to meet certain speed, safety, and emissions requirements. However, the man obviously isn't calling the Blonde a car. Instead, he means that it is legal for him to sleep with her, even though he is 30 and she only 19. In America, you typically have to be 18 or older to sleep with someone else older than 18 (i.e: a 17 year old can't sleep with a 25 year old, but an 18 year old can sleep with anyone from ~18 - 100)
3) This is a play on the phrase "If X, then I'd eat my hat". It is typically used to express that you're super confident X is true (i.e: If this car doesn't cost $100,000, then I'd eat my hat!" or "If that man is any older than 20 years old, I'd eat my hat!". In these two cases, the speaker wants to stress how sure he is that the car is expensive or the man young.). The speaker substitute "eat his last paycheck" for eat his hat but it means the same thing. In this case, it is like saying "I bet my last pay check that she is only just 19 years old".
Hope this helps!