The current merge tags are fine, but very limited and restrictive.
Handlebars (http://handlebarsjs.com/) will allow data to be passed to emails when they are sent and allow total control over data and display - including all data for clients, invoices/quotes down to individual lines on quotes/invoices.
handlebars of course isn't the only template language around, but it is (probably) the most popular.
Sure, it needs a bit of knowledge to use, but equally it's not hard to learn.
