in my experience and from what i have seen in fighters, Boxing compliments wrestling, and jiu jitsu compliments muay thai.
Wrestlers like to keep a strong base, so they would theoretically have a hard time adjusting to the high possibility of being off-balance in muay thai. Boxing is great for wrestlers IMO. I'd say if you have not done a striking art in your life, do boxing, then kickboxing, good pugilistic skill is just soooo important. If i had a dollar for every kickboxer i have seen with sub-par to poor boxing, i would be able to quit my job and train full-time.
Personally i think, Jiu jitsu and Muay Thai are a match made in heaven, they both require flexibility and "good hips" as they call it. Both are very versatile arts, modern jiujitsu encompasses all forms of grappling that would work under they're rules. Muay thai includes of course, all limbs, and it isn't strictly just thai techniques, basically anything that would work in the ring is acceptable in modern kickboxing.
Keep in mind tho, from what i have seen, good boxing skills are an indispensable asset in an MMA match, you could have shitty kicks, or elbows, knees etc. But if you have heavy, accurate hands you will go very far in the sport.
One thing that i noticed is missing in alot of MMA fighters is fundamentals... I always see them swinging for the fences, trying to get that big punch, and alot of them use the "superman punch"(which was originally meant to be a clever and deceptive setup) like it is going out of style. One day i want to see the "art" brought back into mixed martial arts, and i am talking specifically about the stand-up fighting, i have only seen a handful of fighters that are masterful at MMA striking. The striking is lagging way behind the grappling in this sport.