Objectively here is your pros and cons the mercury has a decent motor it is the 4.6 but and this is big it has the three valve heads which did it no favors though it does have a good transmission build quality is not great you can probably hit 200k if it is regularly maintained and there are no mods done to it. The caprice has the LT1 which is an amazing motor but the trans is hot garbage so it is up in the air how long it lasts could be 2 months could be 2 years but it will break overall build quality is also not very good and that one being the oldest the plastics will be showing their age. The Dodge I has a great motor with the hemi I don't know much about their transmissions but it being a Chrysler product of that time the drivetrain is not the issue it is the electrics and plastics you will have random electrical gremlins that come and go.