What does the 20k budget include? If it is only for the purchase and you have the same amount spare for the first two years a 60ies C172 will do. If 20k is your total budget for purchase plus first two years surprises, you will only be able to spend 10k (rule of thumb) for the plane, which leaves you with older C150 and out of luck for a 4-seater. Always keep in mind, owning is NOT cheaper compared to renting, unless you fly well in excess of 100 hours per year.