you said slight black smoke, not an issue for a diesel engine. Did you go through the emission test without any issue recently ? how was the smoke density, carbon particles in the report? Do not panic unnecessarily. However, getting a tune-up done, as in Injector cleaning and re-calibrating probably with a nozzle replacement will always provide better results towards black smoke and pulling power plus better fuel efficiency.
having said that I assume you change the air filter and do services regularly. Clogged air filter can cause a black smoke with a drop in pulling power.
check the diesel filter and drain water or replace if required.
refrain from commenting as i've had mixed experience with different places, go with a reference through your mechanic. However, heard good comments on D*mo but never got a service done through them.
if all goes well and sourcing new nozzles is not an issue, yes, can be done in one day.
injector nozzles, depending on the condition of the nozzles, o-rings
a denso nozzle would cost around 4,000, not quite updated on this.
edit: P.S - Mileage is not the ONLY DETERMINANT factor for an engine overhaul. I've seen 1C and 2C engines running close to 300k without an overhaul, a relative of a friend running his HIACE close to 500k without an overhaul!! Keep the overhaul as the last ditch.