What is the highest mileage i should have when I try to buy a 2008 Dodge Avenger?

you always try to get a lowmileage unit but if it's beentaken care of it may be ok. as cars age and get mileage things go wrong with them

Thank you so do unthink the car will last if it was taken care of?

it will havea better chance yes, but parts do wear out with higher mileage cars.

Thank you do u think it's worth trading in my 97 jeep Laredo with around the same mileage on it for this 08 avenger?

that is something only you can decide.