I have a Discovery 300 Tdi engine from 1994 powering my Defender 110.

Question is - how often should you check the injectors or should you rebuild/renew after a certain mileage ?

I am aware that even a worn injector will ensure the engine will continue to run....

Can you just replace the injector tips ? or get complete new injectors ?

Auto or manual engine - I presume a manual unless someone got very creative but you never know....

It depends entirely on service conditions but I would suspect from 100k onwards is when you start thinking about it. You can buy new injectors for manuals, or rebuild yourself there are a few guides on the web and the bits are available. If you pull the injectors out any decent diesel tech or workshop should be able to put them on a tester and look at the spray patterns for a few quid and advise.