I am fairly new to scrapping but that sounds like a great idea!!

Definitely use acid free paper - you could make the envelopes out of patterned paper to match the rest of the layout. Not sure about the paper you would write on as you wouldn't want it to be too thick. You could 'hide' the envelopes in the layout if they were made of the same paper as your other paper details but you could also hide them behind a photo or large embellishment (like a flower for example). On other layouts they could be an embellishment - i.e. not hidden and you could address them to your DD like proper mail with a little made up stamp etc.

I think journal entries that are handwritten are fantastic in themselves - it really personalises the page. I also like the look of using white or light coloured gel pen on a darker background every now and then to mix it up from straight black ink on a lighter background.

Have you asked for ideas and product suggestions at your local scrap store? That's where I'd start. Also there are lots of internet forums where you could possibly ask about this and pick up some good ideas.

GL. Your idea sounds great. Have fun completing your pages.