How to manipulate on mail template

Hi experts,
when i send the sales quote via email we need to use an e-mail template.

The template should contains default text, data from the sales quote (contacts detail, salesman name etc.) and company logo.

The user need the ability to change/adapt the default text during the process.

When using the default template - the user can adapt the default text but we as administrator can not add the logo, adding data from the sales quote.

When using an e-mail customized template, the user can not adapt the text.

We need to create a template that supports booth : Predefined default text, logo and data from the sales quote and the ability to adapt the defaults by the user.