I've not heard of DPF issues and had a quick search on the forum here and there are only one or two example of "DPF" coming up. There's a regen mode which kicks in well before there should be problems if you don't regularly do long journeys.
I did forget the swirl motor issue which these merc engines suffer from (earlier cars more than later ones as they improved the turbo inlet pipe) though the Custom300c guys sell an emulator fix: Link
Also a few more things to think about
1. check the alternator has had a shield fitted Thread
. The problem if you pour oil in the engine and some leaks it goes straight onto the alternator with the expected (bad news) result!
2. fuel pipes. if you smell diesel under the bonnet check the condition of the fuel pipes that go from the filter in the middle of the vee on the top of the engine and down the side by the alternator (front left i.e. driver's side as you stand looking under the bonnet from the front of the car). They come loose/wear and need tightening/replacing. not a big job. Again they can leak onto the alternator with bad consequences for the alternator
3. turbo outlet. There's a green O ring (link
) which takes all the pressure on the outlet side of the turbo. An easy fix, if it hasn't been replaced you might see an oily residue where the shiny metal pipe comes off the turbo. To see this and (2) you'll have to lift the engine cover off
4. under bonnet wiring. While you have the cover off the engine look at the big thick cable that runs across the engine on the right hand cylinder head cover. That rubs on the aircon pipe which can cause codes to be thrown. If the car's been cared for it's likely this is wrapped/otherwise protected
4. starter exciter. Unlikely to be an issue on your test drive but you'll come across it one day. the terminal on the exciter wire to the starter corrodes and stops the starter working when you turn the key. It's a simple bit of soldering to replace that with a better/newer one to avoid getting stuck (many of us learned about that the hard way)
Sorry lots of small things but worth checking to make sure you get your hands on a cared for car (or get one needing some love for the right price)