I have owned four Town Cars, 1992, 1998, 2002, 2006, and now a 2008. I have never had any major problems with any of them. I would get a second opinion.
Wow yeah, I agree your mechanic might be ripping you off. We had Crown Victoria's and Town Cars with 4.6 engines and never ever had this problem, although I suppose there's always a lemon in the crowd.
I have a 97 Lincoln Town Car Executive.
It is a very beautiful and smooth ride, but I ran into a problem with my intake when I bought the car. I was told nothing was wrong, but I later found out the intake had a big crack down the side of it, where the heater core hose attaches to the fitting, and it had been epoxied. Well I had a brand new intake put on, and I drove the car for 3 weeks, and the new intake cracked down the front of the thermostat house, due to it being thin.
I was told by 4 different mechanic shops, that the intakes are really thin around the heater core fitting and the thermostat housing, so they break easy, due to expansion when they heat up.
Also, as to the part that broke on your car, I highly doubt you broke a valve; they have forged steel keepers and springs. If you lost compression, a ring may have collapsed on number 3 cylinder. All you have to do is pour a little bit of oil down in the cylinder and put the spark plug back in, and start it, and it will smoke for about 15 minutes, and then stop smoking, and the problem should be solved. This is a common problem with the 4.6.