The Navigator and its twin, the Expedition, come in 2 sizes, standard and long. Long is great for families. (My daughter's Expedition EL can comfortably haul 4 adults and 4 kids.) You need to make sure to get the HD Trailer Tow Package is you want to tow anything over 5,000 lbs.
Lincoln had gone through SEVERAL different rear spring setups. The older models used a air spring/strut. These work fine, but is one failed, it was VERY expensive to replace. Then for a few years, they used a hydraulic self-leveling rear spring/strut. Also expensive.
Aftermarket companies sold conversion kit that took you back to struts and metal springs because the repairs were so costly.
A few years ago, they went to a 2 piece rear spring/strut. If the air spring fails, you only have to replace the air spring, not the entire strut assembly. Much less expensive.
Like any air suspension, it is prone to leaks. If I were buying new, or certified used, I would check into the Ford (not aftermarket) Extended Service Plan making certain it covered all parts of the air suspension system.