It would be helpful to know how you are defining "templates." In Word, this term has a specific meaning, which is not what you are referring to. If you mean, simply, messages; and presuming that none of these is either particularly long or has extensive graphics and formatting, I'm not sure what qualifies as silly with respect to the size of your document. Assuming it's 150 pages or so (and growing), with a short message on each page or continuously entered, that should not be too taxing on Word, unless your computer is as old as your software and has limited CPU, drive, and RAM.
I have not done what you are doing, but I can see three simple ways of approaching this. These apply to Word 2013, though I'm looking at 2010 right now and there might be minor differences.
1. What you have indicated, namely, creating a hyperlinked index (Table of Contents, TOC) at the beginning of the document that links to the "templates" within it, just as a normal TOC would link to headings. The way to do this is to add heading text to your numbers for quick identification, make them all Heading 1 style (look up Styles, if you are not familiar with them), or using Heading one for categories and Heading two for identifiers, and then create the TOC as hyperlinks, which is the default.
2. Make each "template" a separate Word document and place them all in the same folder. Create a separate document, make your list of numbers/headings in normal text, and hyperlink each to its document. Ctrl+click on one and you'll get a pop-up about using an SQL something or other; answer no. Your document will open, you can copy the text, and paste it into your email.
3. Make an Outlook message with each "template", enter the heading in the subject field, and save it. By default, they will be saved in your Drafts folder, but you can create a new folder(s) and move them all to it, if you choose. Then, each will be in Outlook already. When you need one, open it and enter recipients. In Options, change "Save Sent item to" the folder in which you keep these, if it is not the Drafts folder, and send it.

Best,
ULodesk