Workflow to send email where To is a field value

I have a SharePoint list where the DateCreated field is empty and when it is populated I need to send an email to a recipient who's email is in the UserId field but I need to complete the email address first:

When DateCreated contains a value for the first time, add "@ac.uk" to the value in the corresponding UserId field and send all field values with custom names to that email address. i.e. I don't want the field names being sent, I want to send custom ones

So if UserId = '8489234', send all field values to '8489234@ac.uk'

For the field named 'GradeAwardedForExp', send it as 'Experience Grade'. (the field names do not have spaces as it used by a PowerApp). The alternative might be to use the field Descriptions instead (DisplayName has 32 char limit so cannot be used in the email as a lot of them are truncated) but I'm not sure if Description has the same length limit.

Is this best done in VisualStudio? Can such a thing be developed in C# or can it be done in Microsoft Flow? I'd prefer to use either custom code or Flow rather than SharePoint designer.

Is it possible to call a REST API to invoke an external service to query the list item to build the custom email body and recipient when DateCreated is populated?

thanks,

Alistair