OK. I think I might have some suggestions, but am not 100% they'll work...

My partner's cousin has cerebral palsy and is confined to a wheel chair I understand that it'd be hard to find one that would work well.

I was going to suggest a Hitachi or one of the cheaper massager type vibes because they're long and the one I have "the rechargeable massager by cal exotics) has a twisty handle. However, that particular one is not insertable, but good news is if you have a Hitachi, you can buy these attachments that you can use on the end of Hitachi. I think SOME of the cheaper Hitachi style vibes are compatible with attachments, but I'm not sure. We may need to consult DreamWolf on this one because I think she has some pretty extensive knowledge in this department. She may come across this and give her advice.

I'm thinking your best bet would be the Hitachi. It's been named the most powerful vibrator on the market for years so you won't have any trouble with it being too weak. It's somewhat long, but it does plug into a wall. I believe the button is a slidey button, similar to what you see on hair styling products, again, I am not totally sure though.

Hitachi link

Also, WetOne has done amazing reviews on some of the attachments, like Gee Whizzard, which is here link

That Gee Whizzard attachment would allow you to use it internally.

Then there's the mini miracle massager, but I think it's really short.
The Hitachi is higher than your price, but not by much.

I hope this helps. If I happen to think of another one that might work, I'll let you know.

PS. Here's another Hitachi attachment that will provide both clitoral and some internal stimulation all at once : link