That would be a very informal letter so I'm not sure you need to worry too much about 'rules' anyway (are you sure the US would finish a salutation with a full stop?).
Hi Peter,
is the form I would think is most common in the UK to match with the more formal 'Dear Peter,'
This American would say:
Hi Nona,
Hello Spinnaker,
Yes, just so, in a business letter, we would use the colon.
Dear Ms. the'Brit:
Dear Mr. Spinnaker:
But the full stop is not right.