* If you trade your car in on another one you will only pay tax on the difference. You will get less money from the dealer compared to selling the car out-right but the savings on the tax will make some of it back plus you will not have to pay to advertise and then wait to see if it sells.
* One other thing I always consider when purchasing a new vehicle is; do I want a monthly payment or do I keep the old car and deal with a repair bill every now and then. It sounds like you have gone through most of the usual issues with the Impala so you may wanna reconsider just keeping it.
* Another thing to consider when buying a car is to buy one that there are lots of them on the road. This way there are plenty of spare parts laying around in wrecking yards and the problems are widely known and can be diagnosed fairly easily. This is why I bought the Grand Am and Impala there are literally millions of these cars on the road. I also learned how to do most of my own repairs to help defray the repair costs.