what is the estimate to repair the alero to a safe driveable condition? You have already spent some money on it it may be worth continuing to fix it.
Taurus can be a nice car. I prefer them though if they have the 3.0 with 24 valves the other 3.0 lacks power for this size car in my opinion. Do you know what engine the cars the dealer is offering as a trade has? I have a friend who was a service advisor at a Ford dealership. He said he saw a fair amount of customers with taurus that got to 200k and beyond. That convinced me to buy my first mercury sable ls which is basically a taurus in mercury clothes. I think the life of a used car has as much to do with how the previous owner drove and maintained it as it does manufacturer. See if the dealer has any maintenance records on the Taurus that may help you decide.