Yes, for this specific scenario, you absolutely can!

Dispel Magic is not Counterspell and can't be used as a reaction, as per the rules. However, in this instance, you are talking about someone having cast the spell and is readying an action earlier in the turn, and on this spellcaster's round (later in the initiative), they cast as their action, Dispel Magic against the readied spell. For this specific situation, it absolutely works! The description of the dispel magic spell says (emphasis mine):

Choose any creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends. For each spell of 4th level or higher on the target, make an ability check using your spellcasting ability. The DC equals 10 + the spell’s level. On a successful check, the spell ends.

So you can choose the creature who has cast the spell, CHECK.

It is a actual spell we are trying to dispel, CHECK.

The rules for Readying a spell state (emphasis mine):

When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration (explained in chapter 10). If your concentration is broken, the spell dissipates without taking effect.

A spell has been cast, CHECK.

There is magic energy an it is vulnerable to dissipation/dispel by numerous means. CHECK.

The spell has been cast, so it is a done deal, the energy has been activated (slot used) and you are holding the spell energy within the caster. There is no way around this very specific fact, by specific over general RAW, the spell has been cast before readying it. There is an active spell, it is vulnerable to being dispelled/dissipated and losing the already cast spell (and the activated spell slot) by other means, those being loss of concentration or holding on for too long, and reaching your next turn. The readied spell completely qualifies for the dispel magic spell.

This has nothing to do with readying a dispel magic spell and using it, this is the other way around, do not confuse the different situations.

The Sage Advice Compendium ruling, already quoted about that different situation, is completely correct, in that you cannot dispel a spell not already cast. However, in this instance the target has specifically cast the spell and the magic energy is active.

To quote the most pertinent part of the ruling:

The main reason is that dispel magic removes a spell that is already on a target, whether that target is a creature, an object, or some other phenomenon. Dispel magic can’t pre-dispel something. If a spell isn’t already present on a target, dispel magic does nothing to that target.

The target is the spellcaster, and there is a cast and active spell present on them in the form of the active spell they have readied. By Sage Advice's own added conditions, the readied spell completely conforms and is vulnerable to the dispel magic spell.

No, dispel magic can't be used this way.

Dispel magic is not intended to be used until the spell has actually taken effect. Consider the Sage Advice Compendium ruling regarding the timing of dispel magic. It's pretty clear that the rules intent is that dispel magic cannot be used until the spell has actually resolved:

Can you ready dispel magic to stop another spell from taking effect?

The easiest way to stop a spell is to cast counterspell on its caster while it’s being cast. If successful, counterspell interrupts the other spell’s casting, and that spell fails to take effect. Counterspell works against any spell, regardless of a spell’s casting time or duration.

With the Ready action, dispel magic can be cast in response to another spell being cast, yet dispel magic can’t substitute for counterspell. The main reason is that dispel magic removes a spell that is already on a target, whether that target is a creature, an object, or some other phenomenon. Dispel magic can’t pre-dispel something. If a spell isn’t already present on a target, dispel magic does nothing to that target. The best that a readied dispel magic can do is dispel a spell immediately after it’s been cast to prevent it from having any effect after the action used to cast it. For example, on your turn you could say something like this: “I ready dispel magic, and if the high priest casts a spell on anyone, I cast dispel magic on the target if the spell takes hold.” If the high priest then cast hold person on your companion who fails the save against it, you could unleash your readied dispel magic and end hold person.

Yes, because the spell is no longer instantaneous

The rules on Readying a spell state (PH 193):

When you ready a spell, you cast it as normal [...] and holding onto the spell's magic requires concentration [...] If your concentration is broken, the spell dissipates without taking effect.

Since the spell is now a concentration spell, it should be susceptible to dispel magic like any other concentration spell—including being dispelled before the trigger.