"It has not been easy to get officials to agree." is correct. It would be incorrect to add "with" there. If you want to add "with", you'd need to say "It has not been easy to get officials to agree with one another", but those last three words are redundant.
This isn't quite correct, in my opinion: "it is also difficult to ask". Asking is not difficult. Getting the answer you want, might be, though. :) "In addition" and "also" mean the same thing, so saying both is redundant.
If the meaning is that buses currently within four years of their retirement will be donated right now, then I would say "It's also difficult to get the bus company to agree to donate buses. However, she has now reached an agreement with a bus company, which will donate buses that are within four years of their retirement (or decommissioning) date."
If you mean that buses will be donated as they reach their retirement age (which I don't believe that you do), then please let me know and I'll re-word it.