His 2003 Town Car should already have a factory stainless (t409) exhaust system - leave it alone for now.
I also highly advise AGAINST any of those HOT air kits.
What you want is an airbox from a 2003-2004 Mercury Marauder from
www.adtr.net, and along with that a performance computer tune from
www.blueovalchips.com
Get the xCal2 flasher, it allows you to adjust parameters like shift feel and timing to your preference, rather than *just* the tuner's settings
The new MAF uses the same connector as your stock MAF, no cutting or anything.
You will need to drill a hole in the bottom of the airbox with a hole saw to attach to your Town Car's stock draw through bottom intake tract. (easy) The airbox bolts right in place of your stock unit, and uses an 80mm MAF instead of 70mm MAF (this is why you NEED the tune at the same time)
Those two upgrades will really help wake up the car.
From there, later on you can add other bolt-ons like Steeda under drive pulleys, a 70mm throttle body, ported plenum and steeper rear gears etc....
If you really want to get serious with the modding, Kooks makes long tube headers for the 03+ panthers which have shown tremendous gains with a low-restriction exhaust. The ultimate would be an Allen Engine Development supercharger kit
CHeck out
www.adtr.net for tons of panther platform performance goodies