Animate a Slider With Json File Import?
Good question Xavier! I had a similar job recently, here's how I did it.

First I would suggest re-working your json file to be a little easier to parse. For example (hopefully I got your intention correctly):

{
    "slider1": {
        "start": 1000,
        "end": 2000,
        "duration": 4,
        "ID": 1
    }
}


Then add two sliders to your layer, one called Anim Duration (seconds) with a default value of 4. If you want to read this value from the json file add the following expression instead:

var data = footage("example.json").sourceData;

data.slider1.duration


Okay now to the second slider (which is the one that actually animates) add the following expression:

var data = footage("example.json").sourceData;

start_value = data.slider1.start;
end_value = data.slider1.end;

step = (end_value - start_value) / effect("Anim Duration (seconds)")("Slider").value; // alternately you can just plug data.slider1.duration here instead of using a second slider...
count = Math.floor(step * (time - thisLayer.inPoint)); // you don't need to use the in-point here, but it is conventient if you know that's where you want to start your animation

if (count &lt; e_value) {
    count;
} else {
    end_value;
}


(if you have any trouble with this let me know, I don't have AE handy for testing so this was mainly done from memory and a snipped I have on me...)