A deposit is usually described as something that improves your chances. It also has a price tag attached, and for a self-employed buyer it is a large one.
Take a vehicle at R320 000 financed over 72 months. With nothing down at 14,5% a year, the instalment works out at roughly R6 679 a month, you repay about R480 900 in total and around R160 900 of that is interest. Put down 20%, or R64 000, and you finance R256 000 instead. The instalment falls to about R5 343, the total repaid to roughly R384 700 and the interest to around R128 700. The deposit has bought back about R32 200 of interest on its own.
The second effect is the one people miss. A smaller exposure against a car worth more than the balance is a better risk, and a better risk is usually priced better. If that same deposit moves your rate from 14,5% to 13%, the instalment on R256 000 drops to roughly R5 139, the total repaid to about R370 000, and the interest to around R114 000. Between the two effects, R64 000 of your own cash has taken close to R47 000 off the cost of the credit.
All three figures exclude the initiation fee, the monthly service fee and compulsory comprehensive insurance, which run alongside the instalment and belong in your affordability sums. Ask every lender for a quotation that states the total cost of credit, and compare those documents rather than the monthly numbers quoted across a desk.