I am pretty close to absolutely sure your engine has valve stem seals... not that there is not stuff in the way and you need an offset pusher deal and a protective cover on the valve when you install the seal... so as to not mess up the sealing surface when you put it on.... but the actual seals are really cheap..
Not knowing anything about the actual condition of your valve stems... that is pretty hard to estimate...
But in general these are very tough engines , if you keep the oil and filters chenged, and it would not surprise me to see you go another 20,50 or 100 K before you needed the valve job...
Those other items can certainly cause smoke ....anytime you have less than perfect burning of your fuel you are going to have smoke... so if the injection is too early then it has a cooling effect on the air being comopressed, which is what a diesel counts on to ignite the fuel... and the fuel has to be in a fine mist.. or it will not burn properly... so the cleanlyness of the injectors outlet... the precombustion chamber holes have to be the right size , pointed in the right direction, and clean.
There is a ' diesel purge ' sold for cleaning injectors which only involves feeding a can of this stuff into the injection pump which might help .... I have not tried it yet... but others swear by it...