First of all: Good luck Sarah.
Secondly: I think I concur with Boxing/Kickboxing and/or Sanda. Muay Thai will do as well.
Karate (Half star and ^ lol) -Not all Karate is created like, so I would just recommend that if you go this route that you go for full contact or knockdown. I only practiced traditional styles (Shorin Ryu, Sanchin) when I was younger, so I don't have the best knowledge of it minus experiences. I have sparred with a few kyokushin guys though, and they are ruff. lol
TKD* (Also not created equal) and Savate^ would be great to develop accuracy in your kicks. The hand strike mileage may vary though. The guy I sparred with it was great all around though.
Boxing*- Definitely gets the job done, and I've noticed a nice mix between it and great grappling like you have. Of course kicking, knees, and clinchwork are neglected.
Kickboxing*? Hopefully in this case your hands and feet would be a good set of weapons.
Muay Thai*- You should already be good to go in the clinch with your grappling, but it might complement well with adding a strong strike game to that place too. Plus your striking in general improves. If anything: I don't know your frame, but if you have short arms and legs, you always have knees and elbows to bring the pain.
Some throwing (Mainly sweeps) may be included as well, but you're already good to go there.
Sanda- I've never practiced Sanda (Sadly lol) nor sparred with anyone from that style, so I can only go off of experiences and personal research.
I recommend it from what i've seen though. Your strikes will be good, and depending on the school, there might be some grappling differences that might add to that part of your game too. I don't know the Sanda grappling well though. I've heard it's wrestling (Shuai Jiao) with an almost Judo flare at times.
Wing Chun^- I've only taken one class of it, but my brother and his best friend (Who is an instructor) practice it extensively along with TKD, Muay Thai, grappling, and more.
I'm considering taken it along with Muay Thai recently (And BJJ). Really I don't think it should be your primary if being in the ring is your goal. It can definitely help with reactivity and countering though. There's some concepts that really help, and I've noticed a few mma gyms here and there (I.e. The wooden dummy, chi sao) that use some of the pieces from it to help. The only problem is at times full contact sparring might be non existent, or something that takes a long time to be done, but that of course depends on the school. At the same time, the style was originally meant for smaller people, so I wouldn't throw the baby out with the bathwater.
I was much less experienced the last time I sparred with my brother, but coming up with TKD/Boxing/Kickboxing wasn't enough when we did. He tore my ass up, but he has many influences.
At the same time though: Even when you consider a person's physical features it's only a guide based on thought, and not a set in stone rule. You don't have to consider you build at all if you don't want, and if you're still unsure, there's nothing wrong with going Martial Arts shopping. You should anyway because a crappy school is crappy school regardless of style and vice versa.
*(I've practiced)
^(I have sparred with someone with a background here)