so are we talking professional monitors or good high end over the head rest ones or cheap halfords jobbies?
if that's all you want you don't need the alpine box at all, if you buy option 1, which as i said you defo need anyway to change the rgb to composite with sound as well.
so you can see it gives 2 video outs and 1 stereo audio out which could be split to feed into any monitors.
when the rns is set to output dvd, it will not be interrupted at all and will work ok for the kids in the back.
at any later stage you can add the OEM camera if you buy a cheap camera your into changing the video signal again back to rgb as the RNS only inputs and outputs RGB signals.
BUT if you do decide to add a camera all the wires must go to the existing plug so some pin swapping is required in the future, if you have never done fiddly car electrics before its not the hardest but can be a pain if your learning.
you also asked what i'm running, well i have rns 510, oem tv tuner and oem reverse camera plus i have same as option 1 in your ebay list for outputting to some rear monitors, i don't have any installed but i ran power and video and sound for if/when i ever got round to it
