Im not sure if its a diy job. Maybe if its just no 1 it could be but I know that to get to injector no 5 you either need to drop the motor or make a whole in the firewall or in my case they did both !
As far as the injectors go I would say don't go for a second hand injector, unless you can have it tested to verify that its not leaking and working within spec.
I also heard / read that the injectors cannot be refurbished but I had all the injectors on my jeep done by the Bosch agents on Main Reef Road in jhb.
They refurbished the injectors by installing new tips on them at a fraction of the cost of new injectors.
This was done about 6 months ago and my jeep is really running very nicely.
The bosch guys also showed me the tests they did on the injectors before and after and they have all the tools to do bench tests.
If you are in Jhb - J&M Diesel (Bosch Agents) 011 474 8955 speak to Hannes Jnr/Snr.
Disco 4 SDV6 SE
1976 Series 3 109 PUP 2.6l
BMW R 1200 GS