There are different situations.
Some tips from own experience and with my clients.
Even you have a big deposit put done nothing or the minimum deposit required because then they generally give a better interest rate then after a month or 2 pump in your deposit and notify them that it's a capital payment, the interest rate won't change and you will save some $$$
Then stay away from ballon payments, unless it can work for you! I bought my car with a ballon because it arrived before expected date & the buyer of my Double Cab was waiting for some money to come in 2 months later. So when he paid me I paid in my ballon, and some capital, which would have brought my installments down, but I opted to keep the installment as is which in turn reduced my term by 15 months.
With the New Credit Act you can pay off a vehicle at any time with no penalties, however if the finance is in the name of a company you get a 3 months interest penalty or you can give a 3 notice to avoid the penalty.
Finally I financed my 335 through BMW Finance and I'm very happy with them. You can try negotiate for a better rate, no harm in trying, I did a squeezed ,5 out of them.
Good luck