The heating pad, battery, and charger showed up a few days ago, and the monkey arrived today. It's a nice size and is quite soft. The only problem I can see is that the weave of his skin isn't very tight, so I will do my best to bury everything within the fluff so it can't be seen or felt.

I think that if I do end up making him purr, the motor will have its own switch. She might not want that feature at all, or might get tired of it. Being warm and purring simultaneously will drain the battery faster, so there's that to consider, too.