The 4.6 is a bit under powered for anything but flat freeway cruising. If your towing, driving through mountain passes or at higher altitude you will find it lacking, but usable. You will just need to downshift a lot more. A diesel will give you way more power and better mileage, but at the cost of increased maintenance expense and noise. The 5.4's are much cheaper to maintain and rarely have issues that are show stoppers. They fire up easily in cold weather, and have mostly adequate power. The V10 has gobs of power, are just as reliable, but at the cost of worse mileage than the 5.4. I wouldn't even consider a 6.0 unless it's been completely Bullet proofed.
Your Craigslist find has higher mileage, but it shouldn't be a big concern assuming it's had regular maintenance. With any engine, there are known issues, most of which are not too bad to fix. The gas motors loose ignition coils and fuel pumps occasionally. The 7.3 tends to need new crank position sensors every few years. In your price range, you should be able to find what your looking for, but it may take a while, and will probably sell quickly so be ready to move at a moments notice before someone else does. Best of luck.........
Agile Ride improvement package