Here is the main things I look for in a '99 model: if it's a Vulcan motor, the X code transaxle (look at door certification label for the X under TR or check the transaxle pan for AX4N stamped into it). Turn the front wheels all the way in one direction and check the lower ball joints. Some '99 models develope a exhaust Y-pipe crack between the cats. And avoid a fleet delete model, these cars do not have a remote keyfob module and makes many things difficult if you want to ever add auto lamps or other mods like some of us Taurus owners do. Simple way to see if it is fleet delete is to look under the dash above the gas pedal for a white box and a relay junction box. If there is open space and no remote with the car, it's fleet delete. This is just my opinions on it. I've got 3 1999 models and one of those has over 300,000 now with no issues ever.
If the engine is code U it is an iron engine. Vulcan
If the engine is code S is aluminum Duratec
digit #8 in the VIN.
Broken springs are common in all years. Fronts more often. They break at the bottom, first break is about 2/3 rd way around the bottom coil. It will still work but trouble soon to follow when the next part breaks and then it all "down hill". You can feel the spring for the first break.
On iron engines, rust and corrosion due to aluminum and iron is the same cooling system. Much more important for scheduled coolant changes. Buying used expect to replace the water pump and flush the system. Changing coolant and PCV valves seems to have been lost in the maintenamce procedures for most people.
Taurus/Sable are rugged, reliable, long lasting, cars as they age.
Mine '87, '88, 90, 92, '95, '01, '03. Never replaced of did major work in engine, or trans. All driven to high miles, then passed on to my daughters and driven on and on. 3 Vulcan, 2 Essex, 2 Duratec.
Oh yes, in the rust belt, they rust just like all cars.