Pays to take someone that knows with yuh donit.
Don't worry so much about the miles on the engines. If maintenanced correctly in the past, a 200,000 mile engine can be just as good or many times better than one w/50,000 miles.
The 5.4L's with higher miles are worth more than the smaller V8 modular motors in a like situation. The 5.4L's normally don't work near as hard as the 4.6L's throughout it's life. In-turn, engine longevity is higher for the 5four. Mpg's are identical for the two engines.