Well I'm happy to hear that you're one of the few that has had a fairly unproblematic 850. My 96 has been abismal to say the least. Common problems that these babies face are rear main seal leaks, bad heater fans and ABS control module failures. As I'm sure you're aware, parts are not very affordable on these and I would certainly entertain the idea of putting something younger in your "fleet" considering their mileage as well. The F-150's are good trucks, but with the 100K+ on each of your vehicles, each is closer to the end of its life rather than the beginning. I am sure you get hosed on the M5 parts as well. I'd keep the F-150, cheap to fix and get something new as your every day car. Resale sucks on the 850, so it's really your call. If anything goes, it won;t be cheap.