The 4.6L SOHC engine, as it stands from the factory, is an extremely stout engine. My Ford tech buddy regularly sees cabs and other fleet vehicles with over 300k on their 4.6s and these fleets aren't even regularly changing oil. (He saw one 200k Crown Vic with the stock air filter!) Yeah, if you don't change the oil for 75k miles you'll get some blowby and you'll be smoking all around town, but they still run.
Even in modified N/A form, its a great platform. How long it will last in heavily modified form depends on the mods, the installer, and the level of upkeep, but if you're concerned about your starting platform, you've got nothing to fear.
SOLD: PI Headed '98 GT - 248rwhp/306rwtq - firstname.lastname@example.org (2.0 60ft 3300+ DA)
'11 TDI Sportwagen - 50 mpg Daily Driver - Not Quick.