Hi Benny,
Welcome to the Forum.
Email is still new enough that conventions are still evolving. It's far from universally agreed that the conventions for regular, non-email business letters will simply be automatically followed in email. Email began as an informal. 'short-hand' medium, and in fact most people still seem to be influenced by this. I don' think it will ever be as formal as non-email.
As regards how to end a business, non email letter, here's what I think. 'Best regards' and 'Kind regards' both seem to me suitable only for a personal letter to a friend. I see little or no difference between 'Best' and 'kind' here.' Yours faithfully' tends to sound old-fashioned today, and is seldom used. By far the most common is 'Yours sincerely'.
So, what to put at the end of a business email? Some people don't put anything at all. Others feel they should put something, as to them it feels wrong to just stop. I feel like that, here on the Forum, that's why I always end by putting 'Best wishes, Clive'. It's not a great choice, but it's relatively friendly and that's the habit I got into. But I'm not writing a business email.
You could just put nothing, except for a nice concluding paragraph. Or perhaps your name and/or the name of your company. Or maybe, you could put 'Regards'. I'd omit the 'kind/best' myself, and I'd only say 'regards' to someone I already know.
In a way, it doesn't matter too much what you put, because I don't think people much care what you write. They don't consciously read it, they see so many different endings and they themselves don't know what to write or to expect.
I'm sorry not to be of more help to you, but the whole subject is still in flux.
Best wishes, Clive