At work we sold our 98 F250 with the 5.4 motor and it had 300,000+ miles on it. The motor was never replaced but the normal wear and tear parts were replaced. It still ran good when we sold it. Proper maintenance will go a long with anything you own.
I did the plugs at 83,000 miles. I planned on changing the trans fluid and filter, fuel filter, oil and filter, air filter little by little..... dont have a few hundred buck to throw at it all at once.....
I'm at 270,000 on my 04 5.4 4x4 (I know, not an 06 but same body style). Still runs great. Only have done regular maintenance and tranny @ 200k or so. I admit though I have set savings aside for a new motor so i can have peace of mind cause I have no clue how much life to expect out one!
With PROPER Maintenance you can easily see 300+ out of your motor. Tranny may not last that long especially since most are not properly maintained but once you get a new tranny immediately start it on a regiment replacing the Trans Fluid completely and it should last a long time.