Welcome to the Oneshift family, enjoy your stay and do join us at our meet ups and live chat room.
Firstly, not to confuse yourself and a easier way to get through your calculations, separate the new car and the used car issue.
You need to know how much is your trade-in value of your Sunny, which I would think its roughly about $30K to $33K, it really depends on the paper value and car body price. Your car will likely be exported than selling into the local open market. You need to know how much is your outstanding loan (if any), you can just call your bank to find out. The difference of the two figure is what you will take back or fork out to top up the outstanding finance.
Once you have this figure, you should know how much cash you have to put as downpayment, insurance, first installment for the new Honda Airwave. I suggest you have some upfront cash for the new car, so that you do not have to loan too much and too long. Also avoid taking high loan on the new car, get cash rebates to cover your outstanding loan of your Sunny because its a very risky thing to do, it may lead you into financial woes in the future.
By the way, is your Sunny a 1.5 EX Saloon or a 1.6 Super Saloon or a 1.6 Super Saloon (Luxury Edition)? If I am not wrong, there is no 1.6 EX Saloon.
Hope this helps you, do post here or PM me if you have more questions.
Hope to see you at our meet ups, Cheers!!!