Most of the earlier (pre-'99) 4.6's were prone to valve guide seals hardening and leaking above 100k miles, or with general age. If you get a puff of blue smoke when you first start it, or if you sit at idle for awhile and then take off (or go WOT after cruising granny-like for awhile), that's usually an early indicator of this. It's not a terribly catastrohpic sort of thing, but it will cause you to go through a bit of oil over time, fouls out your plugs, and will carbon up your EGR passages; you can get away with running thicker oil, changing plugs more often, and cleaning your passages more frequently, but ultimately you'll wind up having to replace those seals, and it's NOT a fun project. Expect to spend around $600 or so to pay a mechanic to do it, or waste a whole weekend farting around with it, if you try it alone - also helps to have a compressor and fitting to keep the valves from dropping.
Also, if it has a plastic intake manifold, see if they're already replaced it with the updated design manifold that has the aluminum crossover. The stock plastic ones were notorious for cracking and spewing coolant without warning, until Ford came up with the revised intake. (Fleet vehicles were covered under the recall, but it may have already expired by now.) A new intake will run you around $250, plus misc. little stuff.
Otherwise, as mentioned above, the 4.6 is pretty much a bulletproof motor. I hate letting go of my 4.6 T-Bird tomorrow morning, as it's a helluva solid motor, but my budget can only afford gas for one V8 car at a time...