I cannot imagine why this would be that big of a deal. We see it all the time where lawmakers take donations from large companies and groups that are against certain other poticians. I don't think the President is much different, and can't seem to think of a strong enough reason to disallow it.
Trump shouldn't have to stop influencing politics just because he is the President. He wants to have good people to work with in Congress. For that reason, he should be allowed to give campaign donations to help these people get into office. It's just another way of being an effective President.
No, sitting members of Congress definitely shouldn't be allowed to accept campaign donations from the President or any other government official. These donations could easily be used to entice the Congress members to take the President's position and not be impartial as they should. These donations could essentially amount to bribery.
The president should not make campaign donations to members of Congress, or any other politician. Our government needs to operate on a transparent basis. The president could be seen as buying votes for legislation by making campaign donations. Leave the president out of fundraising--it will only lead to trouble if the president makes campaign donations to congress.