To address your questions a bit out of order:
2. Yes, LX is base grade, then Zetec and Titanium is top.
Here's a summary from another thread that should be useful for you:
Purchasing MB/MC Mondeo Titanium
1. Worth paying more to get all the fruit? That depends on what you want out of the car. If you just want spacious transport and keep your change then LX is surely compelling. Zetec essentially adds conveniences (auto lights/wipers, dual-zone a/c), and Titanium loads on the tech for maximum posh. Personally, my head was keen on the active safety features of the Titanium (forward collision warning etc) and my heart was lured to the sports bodykit and sunroof, so the price premium was worth it to me, but aside from the safety tech, I've found most of the gizmos I could live without - with the unexpected exception of dual zone climate control
6. I'm not sure why you think the requirement for rear a/c would exclude LX? All grades get rear vents the same. Zetec up does get you dual zone a/c, but the zones are left/right controlled from the front seats only.
3. The main thing you want to look for (try to avoid) is obviously any Powershift issues. The wet-clutch transmission isn't the basket case that the dry-clutch in Fiesta/Focus/Ecosport has been, but nonetheless faults are not-uncommon and expensive to fix. Otherwise there's no clear trends of problems with these cars, they seem to be pretty reliable overall.
4. Headlights are adequate. I personally would like them to throw a little further down the road (both high and low beam), but what you get is probably par for the course. I do have the Titanium with adaptive headlights (point around corners as you steer) which is pretty neat. High beam seems to me to be spread too wide and reflects annoyingly off road signs and the like. Auto lights are fine but I had to turn off the auto high beam function, which was confused by those road sign reflections.
5. Anything else...
Zetec and Titanium get an upspec instrument cluster (Convers+) with integration with the stereo system. It's neat, but the stereo head unit is nothing to write home about (single CD player with aux-in/Bluetooth/iPod/USB). There's no
ICC or screen with inbuilt GPS or reversing camera etc.
LX's 16" wheels and tyres with their deeper sidewalls give a more compliant ride over rough surfaces than do the 17/18" alloys on Zetec and Titanium. Mondeo is pretty low-slung and the sports suspension on Titanium makes it even lower, so bear in mind if you regularly have to traverse less-than-billiard-smooth roads.
Wagons come with a space-saver spare wheel, but there was a factory option for a full-sized spare (raises the floor of the boot by a few cm). Something to look for if it's important to you. The dish will accept a full-size 16" wheel and tyre but you might fall foul of the bigger wheels on Zetec and Titanium.