Hi the best people to go see for injectors are United Diesels, they are brilliant with advice and will give you options, as for doing it yourself if you have never tackled something like this before I would use a garage as the injectors go through a rubber seal on the rocker cover which will also need changing, the seals are cheap TPS but they will probably suggest you need new fuel feed pipes for the injectors which is rubbish as if you have someone who knows what they are about and wont over tighten them the old pipes will go again, from a personal point of view with that in mind your milage I would change all four as these are Piezo injectors and are very delicate and precise in operation, the old type of injectors had a detent ball and spring arrangement which when the engine was revved would bounce and overfuel hence black smoke, Piezo on the other hand are controlled by electrical current to make the Piezo slices expand and contract and never over lift the pintle this is supported by extra Venturi that are 0.6 which is finer than a human hair and these allow precise addition or decline of fuel up to five times in any given cylinder cycle therefore creating almost perfect fuelling, I have used Dipetane for a few years now, what a wonderful product, as its a hydrocarbon it absorbs water from the combustion process and burns it.
Steve.