If you part exchange it you won't get any more by tarting it up, quite often they are just taking an old car off your hands as a favour if it is 10 years old or more. They would probably sell you the car you want for the same price whether they take your old car or not, so you might as well sell it privately. It is only worth tarting it up if you can do it without making it look tarted up. By all means replace any broken parts if you can get them cheaply second hand. People expect old cars to have picked up small dents and chips.