Haven't read through all the replies so sorry if there's some things already covered:
It is recommended (I would, along with many users) to install ML onto the camera, read through the manual the team provides and get used to the way the camera functions with it. Without manual control over even the basics of Aperture will be a pain, and people have dealt with that by disconnecting the lens from the camera slightly by just rotating it. This endangers the lens from falling off accidentally.
Image stabilized lenses can help if you don't have any rigs at the time and are going hand held. But I suggest against the use of IS while using such lenses, especially for narrative productions, as the IS is built for photography and not optimized for video use. It will yield stable shots, but when there's some movement, there will be "jumps" and very unnatural movement while the gyros figure out how to get a stable view. Doesn't feel cinematic and almost like Post processed stabilization sometimes. Perfectly fine for event coverage or on-the-go journalistic/broadcast work though.
Yes you would need another screen especially to save your back (or gear) by having to get smack behind the camera. Just having a better angle for monitoring helps, but I don't have any suggestions, there are several well rated ones for around $200, so really anything above that in price range should be good, just depends on your needs, and weight should still be considered when traveling even tough image quality is a priority.
For sound recording using an external to record directly into the camera is standard practice, but I record both in camera and external and sync in post. In case the external has a problem or the recording just didn't turn out then there's the in-camera to fall back to.
Sliders - I've been looking at getting a better one myself, and trying out one in a shop I will likely go for the Edelkrone Slider+ as it's super smooth and stable, short for easy transport too. And is pretty cheap compared to most other sliders that are usable. Otherwise you should look at the Kesslar slider options, some are very good for the price as well.
Back with the 5D Mark II the ISO settings were a bigger deal, now with being the low light beast, 5D Mark III noise isn't much of a factor to be concerned over. Naturally dynamic range will drop at higher ISOs, and as it was said above as well starting at 160 and going multiples of that should be good, but I don't think following that method is important anymore since the 5DII. You should stay between 200 ~ 800 for most shooting, while up to 3200 can yeild very usable/acceptable results. You can go beyond that, but the already soft image of the faux-1080P of the 5DIII will start to show more.
Again, don't worry about the noise for this camera, 1600 and below to keep it simple though.
Your iMac - It should do fine, well, depends on your RAM, video card and processor but I'm using a 4 year old MBP and it does fine. But that depends on what's usable, if you're on a tight schedule then get yourself a Mac Pro and upgrade it as much as you can and even then you would want more speed
If you're coming from RED and Alexa then you probably worked with better codecs and compression, maybe even RAW. 5DIII outputs in MOV with .h264 and although works fine will slow down any system as is, you will benefit from transcoding to ProRes first then editing, but again coming from better systems you won't be able to do a whole lot of grading in post. For recording just watch out for those blown highlights, but I suggest slightly overexposing the shots in general as the sensor is better with maintaining highlight detail over shadows.
Now also mentioned there's a firmware hack out that allows for Cinema RAW output from the camera, and again is still in ALPHA and not fully functional/available yet. And your current card will only yield you a matter of minutes with RAW anyways. But, get more cards, at least have more than enough recording room, I wouldn't want to remind myself about the lack of space and just not shoot potentially usable material.
Finally, there is no ML for the 1DX, but the video function of that camera is well regarded, just not "the camera" for video users so I don't hear so much of that, so there will be less material out there on shooting video with that camera.
And batteries, get plenty, in video mode (just having the screen on even) DSLRs just guzzle battery juice