Firstly, it largely depends on the context of the work world you are entering.
If the email is written on behalf of an international organisation, or a Government Department, for example, you can expect the tone and content to be quite formal.
The creative and artistic worlds (and the tech world, come to think of it) tend to be a lot more informal. When I was an academic, the academic world also tended to be relatively informal in how they expressed themselves in emails.
Then, there is the matter of who you are addressing in your email; are they superiors, subordinates, colleagues, or others outside the organisation. Are they friends?
Personally, I would always err slightly on the side of a more formal tone rather than less formal tone in emails. I think it looks better, and - again, speaking personally, - I loathe the assumption of close friendship in breezy and overfamiliar emails. Words matter.
In my case, I will always start a professional email with 'Dear Mr/Ms/Dr/Mrs' followed by the person's surname - and, depending on the context, (which is whether or not I actually know the person or have had personal dealings with them, and whether they are at my grade or above or below that), I may use their first name after the salutation 'Dear'.
As for closing, I am a fan of the old 'Kind regards'; if the person has been helpful, I will thank them warmly - nobody minds receiving a proper acknowledgement of what they have done, even if it is their job.
Re informal emails, even in a professional setting, I will send those, but only if I have come to know the person reasonably well; this kind of informality tends to be confined to people who would be at your own grade, or under that.
Hope this helps somewhat.