You may want to consider waiting for two or three years.
The trade in value for a 7 or 8 year old unit is about as good as it is for a 5 year old unit. Therefore, if you could use it for two or three more years, and get about the same amount as a trade in, then you will in effect use it for free over the next 24 to 36 months. Just trade it in before it is 10 years old or older to get the best trade in. That is simply because banks usually don't like to finance units over 10 years old, therefore the values drop quicker due to the fact that it would need to be a cash buyer.