Injection pumps cost close to $1K to go through, injectors about $60 a piece. So, you decide how much you can afford to buy it for, then spend on it.
When I buy a non-running tractor, I never assume it just needs a "good servicing".......especially a diesel that will not run. Sometimes it's cheap to fix, sometimes it's not. I pay bottom dollar unless it runs, that way I don't get burnt. The difference between running and not on a diesel could easily be more than $1000. Just my