You have a couple of options.
1. Pay a template developer to create a template which contains a userform which is displayed automatically for new documents where the sales staff can pick the relevant components and click OK to assemble the customised proposal which can then be further modified by your sales staff.
2. Buy a product that has been created for this purpose. These links will give you an idea of what software might be available to choose from
https://www.trustradius.com/categori...posal/products
http://www.capterra.com/proposal-management-software/