Yeah, the OE Ford headlamps will get you about 7 - 10 years if properly cared for. It also depends on where you live. If it's a place like Florida, expect less if you don't have a garage. The sun eats stuff up down here for lunch. And BTW, you WILL not get the OE Ford headlamps for cheap anymore., at least not from a good distributor. The reason for this is that Ford's stock of Fox body Mustang headlamps are running low so the price has gone up considerably. See, I WAS going to get the 6 piece set with the Ford headlights but by the time I got to it, the price had skyrocketed. You USED to be able to get the 6 piece set with economy side markers and park lamps with Ford Headlights for $129. Used to I say. Not anymore. The economy headlamps are made by a company called DEPO. The headlights are not too bad but i'm not real sure how long they last. I bought the 6 piece economy kit with econo headlights for $99 when it was on sale in April last year. The headlights are still looking clear and glossy. The center park lamps are yellowing yet still glossy. The yellowing gives me concern because they were not that way when I first installed them.
Anyway, if you are on a budget, get the 6 piece Economy set from LRS. It's a good buy. I just don't know about those center park lamps.
Adding silicone sealant to the sealing areas of the headlamp lenses does not help. You WILL still get condensation in them. I've had to remove my headlamps twice in the last 11 month because moisture formed inside. It's unavoidable here in South Florida where it's damp and hot mostly. I removed them to let them air out. What you really needto do (and what I've heard from a 'few' other people) is to drill a couple of small holes in the bottom and top where they cannot be seen. That way the lamp housing can breathe and circulate air. If you look, you'll notice small holes on your park lamp assemblies. Made for the same reason. Don't know why they didn't put them on the headlamp housings.