There is a LOT on this forum about Mods for the CRD.
In my openion, the main things are:
GDE Tune - This gives power and better mileage and resolves the swirl motor issue. You will love it.
Elephant Hose MOD (EHM) - to take care of the little oil leak where the fresh air intake goes into the turbo. There are several ways to resolve this, EHM is one of them.
The rest is optional.
2007 WK 3.0L CRD, Quadradrive II, GDE EcoTune, DPF-Delete
4xGuard Front, Rear, Belly, & Side Rails, Factory Skids
EHM MOD, Garmin Mechanic EcoHD, K&N Filter (Deleted)