Clients can order products with or without installation. We send out prepayment emails and depending on if they chose an installation or not there will be additional text or not. Installation is a product line in sales order and has it's own product category. 

So in my mind it would look something like this (as i'm a beginner in odoo i don't know if it's even the right approach)

Hello

"text in email"

% if sales order has an installation product/category:

"text for that information"

% endif