Hi,
So get a Tandy model 100 or, even better, a Tandy WP2. Days to weeks on a set of AA's with use, months to years(!) off/standby. Even interfaces with Amigas using Term etc.
I have worked with such things in the past, but to be honest I find the screens are usually a limiting factor. Perhaps I have just become spoiled with good text editors, but I find it difficult to judge the flow of text and the general feel of things with a display that is only a few columns tall.
I'm finding that an old worthless laptop with DOS, a small hard disk, and a text editor installed is a really good tool for the job. It doesn't touch the friendliness or power of a good editor on the Amiga, but the fact that it is portable makes up for that somewhat.
David