I would get the oil tested, it will cost somewhere around £20-30 - if the test indicates no metal wear then you know that engine itself is ok, and problem is with the auxiliaries - and it will prove if the car was serviced properly or not really.
For the auxiliaries, worst case scenario would be: http://www.ebay.co.uk/itm/NEW-ALFA-R...item2a4f02766d
- £300 for the turbo, another £300 for the manifold
Replacing the manifold is a fairly big job, as it requires removing the fuel pump which means that timing belt has to come off, so you are looking at around 8h of work (it should be under £800 for manifold + labor, timing kit with water pump is another £130).
Blocked manifold is unlikely to cause P0237 though.
Reality of the manifold problems is that it is highly unlikely to be main source of problems, but it's hard to prove it one way or the other, so people seem to be using it as excuse when they are not sure what the real problem is.
Don't bother fixing the EGR, just blank it off - cheaper and more future proof.
Map sensor could possibly be a cause, but it's easy to swap and relatively cheap so it might be worth giving it a try (if you pay for the postage i can send you one that is known working and only reason why i've changed it is because i went for more boost than stock 3Bar MAP could handle).