Engine - Injectors, Turbo, fuel pump, timing chain tensioner and possibly the chain itself for peace of mind (bearing in mind it'll be around 250k at this point) Exhaust system, maybe stainless. Replace hoses and vacuum pipes and possibly ditch the viscous fan seeing as it's power and economy sapping!
- OM613 is bullet proof. 250K mikes is nothing! Test the injectors and get them refurbed. Check free play in turbo...and keep servicing it.

Hoses and vac pipes, good plan!
The mechanical fan is removed on mine
Gearbox - Full rebuild, clutch packs, pump etc
- Again, the 722.6 will last 250K miles. Is yours actually faulty? If not, ATF and filter (including draining the TC)
Differential - Remove/Inspect replace parts if necessary
- Unless it's noisy, just change the oil.
Suspension - Replace everything. Subframe bushes, arms etc etc. All OEM parts.
- Yes, that will make the biggest difference in the ride and handling. Also consider all the engine and gearbox mounts and flex discs.
Whilst sub frames are dropped replace all brake pipes and SLS lines?
Good luck anyway! Knowing what I know now, I'd buy a 2009 211 320cdi for £7K.