There have been three generations of the Navigator. The first lasted from 1998 until 2001. During this time, GM introduced the Cadillac Escalade and GMC Yukon Denali, two luxury SUVs that stole a lot of market share from the Navigator. The Navigator was considered one of the first luxury SUVs.
Many luxury options were added for the second generation. Whereas the first generation shared many components with vehicles made by Ford, the second gens were almost all original Lincoln luxury. The V8 engine made 300 horsepower and the 6 speed automatic transmission made it a very strong but peaceful driver. Some features include tire pressure monitoring, touch screen navigation, THX audio, and DVD entertainment in the rear.
The third generation introduced in 2007 saw the addition of the longer wheelbase Navigator L. This vehicle would compete for the same market as the Escalade ESV. The new independent suspension is much improved over the previous models, making for a smoother ride. Although the engine is still the same, the hood allows for greater clearance for bigger engines. The Ford Boss with 425 horsepower will be an option.
You can find these on the used market if you know where to look. Try using eBay to find some great deals and a plethora of aftermarket parts.