Although I am a professional DOP working on RED, Alexia etc, I now am doing a job for the first time on DSLR ...5d3, 1dx Zeiss 14 2.8, Zeiss 35 f1.4, Zeiss 135 f2 (on its way)
I shoot lots of video documentary material using the 5D3. Despite your experience, a video DSLR is complex. You'll need to practice all the modes and "switchology" ahead of time.
If you shoot any hand-held or monopod material, an optically stabilized lens is important. I use the 24-105 f/4 and 70-200 f/2.8 IS II a lot; they are both good. The 70-200 is big and expensive but the images are beautiful. It facilitates getting shots of opportunity. If you're doing mostly scripted narrative work, that's less an issue.
Lexar 1000x 32gb UDMA 7 card
You will need at least three 32GB 1000x cards -- per camera, maybe more. That's for 1080p/30 IPB video (the most space efficient type). If you use ALL-I which takes about 3x the space, you'll need more.
I'd also suggest a USB 3.0 CF card reader. I use this one; it works well: http://www.amazon.com/Hoodman-Ruggedized-Steel-Superspeed-Reader/dp/B005UEB6OK
Make sure your PC accomodates USB 3.0. If not USB 2.0 will work it's just 3x slower.
Despite all the talk about lower-compression formats, I find that IPB looks very good and avoids the complexity of All-I, HDMI external recording or raw video.
Batteries: besides the CF card, you'll need several LP-E6 batteries. If you get a Zacuto EVF Pro, it uses the same battery. For two DSLRs, I'd suggest a total of four extra batteries minimum, preferably six. Add one more for the Zacuto EVF, which (unlike the DSLR) will last all day on one battery.
A three year old Imac for editing.
I use Premiere Pro CS6 on Windows 7; my PC is about three years old but is a quad-core 16GB machine at 4 Ghz with a GTX-660 video card. Your performance will depend on what editing software you use.
I will be going to small villages and shoot lifestyles of people as artistically as I can. Mostly decent light, occasionally low light. I will shoot lots of interviews with audio. I have a focus puller and lighting assistant.
You'll need an external mic, whether recorded in camera or external. I've used a boom-operated Rode NTG-2 shotgun to both camera and external Zoom recorder, also Sennheiser EW 100 ENG G3 wireless lav: http://www.bhphotovideo.com/c/product/618735-REG/Sennheiser_EW_100_ENG_G3_A_Evolution_G3_100_Series.html,
and the cheaper Canon WM-V1 wireless bluetooth lipstick mic: http://www.bhphotovideo.com/c/product/751267-REG/Canon_5068B001_WM_V1_Wireless_Microphone.html
After using all that, my preference for interviews is the Sennheiser G3 concealed in a clipboard which the interviewer holds near the subject. Camera is backed off from the subject using a 70-200 f/2.8 at 200 mm @ 2.8. This is not a covert interview, the subject knows both mic and camera are there. However getting the camera out of their face gives more natural response. Having the mic non-visible (vs a boom-operated shotgun in their face) also helps. Camera is typically hand-held (for brief interviews) or monopod-mounted.
1. Do I need another monitor besides the LCD on the camera to check focus etc?
Yes. You can either use a field monitor or LCD. I've used both and my preference (by far) is the Zacuto EVF Pro. It's a professional-quality tool and has adjustable diopter, focus peaking, zebras, one-touch zoom, etc: http://www.zacuto.com/zfinderevfhttp://store.zacuto.com/z-finder-evf-pro/
You'll need a "Gorilla plate" and LCD mounting frame. Talk to Zacuto; they'll help you out.
2. Which is the best slider for price for DSLR?
I use the 40.5" Kessler Stealth slider; it has adjustable tension and is well made: http://www.kesslercrane.com/product-p/stealth_standard.htm
3. Which is the best way to record audio? Do I record on camera or external recorder and sync using a slate? I have a BeachTek DXA-SLR available for free. Is it any good?
My colleague uses this on his D800; he likes it. For simple productions I see no problem with recording audio to camera input (using an external mic). For multi-cam shoots you only need one camera with primary sound, on the other cameras the onboard mic is OK for sync'ing to that.
DO NOT use audio automatic gain, set input gain manually. Monitor all audio with headphones.
Note the 5D3 audio controls are tricky: when not rolling, the thumb wheel "click" controller adjust it. When rolling, the thumbwheel switches to a touch-sensitive, accessed via the "Q" button. Practice ahead of time.
DSLRs typically record one stereo audio track. If you want to record ambient sound plus the interview vocals you'll need to split this to L/R using the BeachTek or have another recorder.
For syncing audio/video or multi-cam audio, I've used PluralEyes; it's not perfect but helps: http://www.redgiant.com/products/all/pluraleyes/
4. Should I use Magic Lantern software or original Canon RAW? If magic lantern, then which version?
ML is not available in production version for the MK III. It's a great product but I would not use the current pre-alpha version for critical work. I definitely would not use raw video (only available via pre-alpha ML).
5. What is the ideal ISO for video? And if not ideal, what is the acceptable range for noise free work.
If you need to get the shot, you can use fairly high ISO. I have shot wedding and documentary video at 12,800 and it looks good. You'll need to make test shots in the field to verify they're OK for your standards.
6. Is 1/50 the only shutter speed to work at or it does not matter?
Your shutter speed should be the closest available speed to 2x the frame rate. E.g, 1/50th sec for 1080p/24, or 1/60th sec for 1080p/30. Not using this can cause a strobing effect like the opening scene of Saving Private Ryan. See 180 deg. shutter rule: http://tylerginter.com/post/11480534977/180-degree-shutter-learn-it-live-it-love-it
7. Will my Imac be ok for the edit? I will not buy an Imac now until it is refreshed, but just want to prepare myself on the speed of edit.
As stated above it depends on your editing software and iMac memory/disk config.
You'll definitely need a variable-ND filter or drop-in filters on a matte box. I use the Tiffen variable ND filter; it works well. However it conflicts with the lens hood, so if you need both a 3rd party lens hood or matte box is needed. Note the ND filter is mandatory. You cannot shoot video outdoors at wide aperture without one, unless you vary the shutter speed which isn't desirable.
In general the best camera mode for video is manual, with auto ISO enabled. This gives some auto-exposure freedom, yet locks the shutter speed for the 180 deg. rule. Do not shoot in programed mode or aperture priority, which will allow the shutter speed to float.
For slating the shot, I use the Movie*Slate app on an iPad. It is very sophisticated and can do many things, inc'l upload the shot list to your PC and optional wireless timecode: http://www.movie-slate.com/
Slating shots works best for pre-arranged setups. It does not work for quickly-emerging shots of opportunity.
For fluid situations if you slate multiple cameras and start rolling, you can easily burn up lots of battery and memory waiting for the event to happen. DSLRs will time out after 30 min which breaks the take, which squanders the slate's synchronization benefit.