A 13 years old Ducato with 86k on the clock, unless its a motorhome, would fill me with suspicion.

Haven't seen blow by like that for years, i'd suggest excess bore and/or ring wear, the oil is being thrown off the camshaft not the crank and is quite normal but its being forced out by that excess crankcase pressure too.

Its possible an injector seat is blowing by, but unless the injectors are pulled out and the seals inspected thats unknown, no idea if injector seats are an issue with that engine, valves shouldn't be causing this unless its actually done 386k and as worn as everything else (unless its seen no oil changes and possibly allowed to run low on oil regularly), i would expect the EGR to be jammed solid with oily soot and the intercooler if it has one full of oily residue.

If it was my choice i'd want the thing cheap because there could easily be several £k's work to get it back to clean running, on the other hand so long as it passes the emission tests and you keep it topped up with oil it could go on like this for years yet, if you do buy it at the very least drain all the oil out let it stand and inspect the oil pick strainer (endoscope or sump off), if an injector is leaking then the extra carbon in the oil can congeal on the oil strainer causing oil starvation and engine seizure, if the strainer is blocking up you have no option but to fetch those injectors out.

After looking at the video again, it appears the injectors are seated externally of the crankcase, a design i prefer and always have because blow by is obvious and doesn't cause sometimes catastrophic carbonising of the engine oil i referred to previously because any leaks are not inside the crankcase, sorry to have misled you there.

However you still have blow by and yes it is quite bad, it might be worth investigating if there's a crankcase breather on the engine that has become blocked, that pressure has to go somewhere and by the looks of the oil gathered around the rocker cover its possible that pressure has blown the cover gasket.

The injectors might have a seeping leak but i doubt the seals have gone because being external you would see blow by from their seats, being external should make them an easy removal for seal replacement anyway, the first thing the engine needs is a good steam clean to see where that oil is coming from.

Value wise i haven't a clue, as said before if it runs well and you've seen no smoke or excess fumes then it could well go on for ages like this, its obviously using a bit of oil as freely admitted by the current owner, whcih wouldn't worry me unduly.

Have you confirmed the mileage is correct because @ £2k it isn't a motorhome and unless its been a builders van used to reach localish jobs the mileage is low for a 13 year old van.

This question would be better asked on a van or make specialist forum where the engine in question would be known to the posters in the technical section, we can offer a general opinion here but you can't beat having specific knowledge of the engine in question, ie they'd know if there was an engine breather or similar that should have been changed.