I agree -- I think the typewriter is the easiest solution. And she probably has a preference between electric and manual. I know my mom loved the speed and ease of an electric typewriter over the manual. Better to stick with whatever she's most comfortable with.
But for her regularly-sent mail, consider...
1. Having those frequently used addresses pre-printed for her (e.g., on those Avery adhesive labels you can buy and use a template to print yourself or have done at a FedEx/Kinkos).
2. Getting her a set of labels done with her return address -- maybe even something cute and tailored to her style -- from places like Etsy.
3. Buying her a set of
$1 or custom rubber stamp kits with recurring phrases/sentiments she might use in her cards or even a stamp of her
$1