FDII will have a different front bumper facia (which increases approach angle in offroad situations).
FDII Will have 215/65/17 tires vs 215/60/17 tires.
2011+ Compass 4x4 models all use the same springs so FDI and FDII springs are the same (this is why 2011+ 4x4 models are higher than previous years)
The extra ground clearance comes from the taller aspect tires.
FDII package incudes higher driveline vents, this means that if you choose to go through deep water you have less risk of getting water into your rear diff power transfer unit ect.
FDII package includes a 140amp alternator vs the 120amp.
It also comes with factory skid plates, as well 2 front tow hooks and 1 rear tow hook.
In 2014 the Mk's got a 6 speed automatic transmission. However the FDII models still have the CVT Transmission.
The biggest difference is the Brake Lock differential (BLD programming) on the FDII. And the lower overall Ratio of the CVT and axles
In simplest explanation its brake traction control, however it only cares about the speed of wheels across a single axle, meaning if you have 1 front and 1 rear tires in the air, it will selectively apply the brakes on the wheel in the air and transfer power to the tire contacting the ground.
Since the 2.4 is not a torque monster the overall axle ratio (8:125) and the lower ratio CVT help in offroad situations when you need as much torque as possible. Having said that the New auto while not having as low of a ratio would still be fine if you aren't getting too crazy.
It's not replacement for a real locking diff or limited slip unit, but it increases the offroad capability a lot.
The Non FDII Compass' have the BLD but its not as aggressive so it is not as effective.
Long story short FDII parts you CAN add to a non FDII Compass
FDII Front Bumper
Things you CAN"T add
ABS programming for BLD's
Low range axles
Here is a video showing the BLD in action (it's my compass)
Lifted Jeep Compass Hill Climb - YouTube
Lifted Jeep Compass FDII Offroad - YouTube
The BLD makes a large difference offroad, as well the different front bumper allows to go up steeper hills without contacting the ground.
If you aren't looking to do anything to insane. Adding a lift and skid plates will allow you to go down some trails you may not have been able to go down before. After my lift I noticed I bottomed out far less. The skid plates give you piece of mind so that should you bottom out you won't damage anything.
You can't add the FDII BLD programming which really transforms the Compass
If I was you, I would suggest Skid Plates, lift and a set of 16" tires with some 225/70/16 tires. If you wanted to run wheel spacers 225/75/16 would work as well.
Since you have a limited with 18" tires, should you choose to not get different wheels, 225/55/18 Yokohama Geolander AT/S All terrain tires would fit on your factory rims.
Let me know if you need anymore help. I've modified my Compass greatly and really pushed it offroad and would be glad to help you in any modification questions you have.