There are a few things to keep in mind before you start planning about your content: make sure to include any legal requirements such as statements, company registration or VAT numbers.
Detail your required text content in a way that keeps things simple and succinct.
If there is a requirement to display association or certified logos that support your activities, use unified colors such as black, grey or some form of tint.
This will help others to amalgamate into your design layout while still allowing your brand to take fondness.
Go for the look to create a guide template for formatting your actual letter content that will overprint internally.
If your brand logo fonts are not available on your system adopt something of similar and suitable style as opposed to defaulting to Arial or Times.
You can consider opening the letter content with a large short statement or greeting that can help create some focus and break up the formalities and constant scale of the body text.
Another option can be adopting a watermark. Watermarks are best reserved for single sided designs and are typically printed at around 5% strength.