I agree the f150 isn't designed for a plow, but more companies are designing light duty plows for them because of the demand. I have personally seen all 3 of the plows listed below in action on supercab, 6.5 foot beds, with 5.4 engines. The trucks all handled them just fine, and didn't squat down much when they picked up the blade.
Your comment about "leaving the plow on" scares me a little. That's why new plows have Quick-mount systems. They WANT you to take them off when not in use. I can say from experience that carrying the plow around all the time causes as much or more damage and wear to front end components (springs, shocks, ball joints, tie rod ends, wheel bearings) as actually plowing. Also, the engine and transmission are working harder to carry that extra 400lbs, and you're losing alot of air-flow to your radiator.
Agreed, ALWAYS COME TO A COMPLETE STOP BEFORE CHANGING DIRECTION, and always wait for the truck to completely go into gear before hitting the gas pedal.
Even though Ford doesn't recommend plowing with the LIGHT Duty F150, if you do, there are some things you'll need to add if you don't have them already.
1. Transmission cooler, (very important)
2. Tires that are rated for the extra load (the stock 235-75-17 Hankooks with 35psi are not gonna cut it)
3. Counterweight in the rear end. The truck will pick up the plow fine, but it takes alot of weight off the rear end. This negatively affects the safe handling of the truck during steering and hard braking.