It is no ruder or more courteous than 'First' or 'To begin with' or 'In the first place' or 'Let me start by saying'. They are all perhaps a little abrupt.
It doesn't really matter which you use. What does matter is your tone of voice and other non-linguistic cues you provide.
Do the needful is Indian English, which has been covered on ELU.
If you're only interacting with other speakers of Indian English then feel free to use it, but avoid it in any other contexts (most Americans and Brits will think it's quaint/uneducated).
In general, the "standard" form is do what[ever] is necessary, but in OP's specific context most likely nothing like that would be used anyway. If you've just asked for an email address, it goes without saying that you want the other person to do whatever is necessary to give you that information.
I may be wrong, but I have the impression that for many Indian English speakers, "Please do the needful" carries a subtext of "This problem is too complex for me to understand or resolve myself, but I have complete faith that you will be able to deal with it, because you are very skilled in such matters"
As I said, Brits and Americans wouldn't normally use any equivalent for such a trivial problem as finding someone's email address. But if the request was for something more challenging (and crucially, if it was from a manager to a more junior worker), "Do what[ever] [you think] is necessary [to solve this problem]" might be perfectly normal. The implication there is that the manager is authorising the junior to do things he might otherwise not be "permitted" to do (in effect, the junior is being temporarily "promoted" for the duration of the problem-solving).
In that context, it should be clear that (to Americans or Brits, at least) any such phrase would probably be considered offensive/cheeky if addressed to an equal in the workplace (if the person asking isn't senior enough to confer temporary authority on you, they shouldn't be speaking to you that way).
Best Answer
Yes, it's rude. "Whatever" expresses indifference; often, expressing indifference is dismissive, and in this case, it's dismissive of what the other person has to say. Semantically, it's equivalent to responding with "I don't care".
Being dismissive is what makes it rude. Since "whatever" can express indifference without being dismissive, it's not rude in all situations. If a friend asked me what I wanted to eat, and I replied "whatever" in a non-sarcastic tone of voice, it probably wouldn't be taken as rude. Rather, it would express that I had no particular opinion and that I'd eat whatever they wanted to eat.