When shooting speed, always know that you need more light. As you record higher speeds more might is needed to compensate.
Want clean images. Use a lot of light.
You can get away with cleaner images by filming 3K scenes instead of filming it all 2K. Also, consider that you will be cropping the sensor so your wide lenses will get tighter. A 50mm at 4K is a 100mm at 2K, etc.
I did a shoot a while back at 2k/120fps and it was super (and I mean *super*) smooth. We shot outdoors on a sunny day so light was not an issue. You're far better off to shoot high speed on location than to try and slow it down in post. That's where all the things you want to avoid will start creeping in. Take note of the things already said though, all very pertinent to your situation.
Not sure what you mean by "smooth enough"? Why would 24p be smoother than 25p? The higher the playback frame rate the smoother the motion. In addition, anytime you spread a slower frame rate across a faster i.e. 24>25 or 24>30/60... it becomes less smooth because frames repeat.
The only reason to shoot 24 over 25 would be international deliverables or film out.