[Skinny Shooter]

Ok, I believe I know the basics on the mil dot; in that after estimating the range , one would use the same rect. for hold over and windage after consulting drop charts.

Yes, but you also use your target turrets for ranges that your mildots don't compensate for.

But I'm not clear on using MOA. . .. Using Clicks ? to adjust MOA? Using a rangefinder, One would dial in the drop AND windage to zero out the rect. on the point of impact without actually holding over??? is that right?

Yes, you will dial in your come-ups or clicks so you can aim dead-on.
MOA (minute of angle) at 100 yards is equal to approx 1 inch.
Most scope adjustments move the reticle 1/4" for each click at 100 yards. Some scopes move the reticle 1/8" and the scope manufacturer will tell you either way.
So if you turn your target turret 4 clicks at the same 100 yard target, you have just moved your reticle 1MOA.
Those same 4 clicks at 100 yards just moved the reticle 1 inch.
Here is where some folks start to get confused.
At 500 yards, those same 4 clicks of the target turret move the reticle 5 inches. And that is still one MOA of movement. At 1000 yards, that same 1 MOA (4 clicks) of movement will move the reticle 10 inches

If I outlined the Moa system correctly, in chosing a recticle, One would only need a cross or dot. , you wouldn't need hold-over dots and windage dots could be used or just dialed in.

That is correct, you could just make the appropriate click adjustments on your turrets and aim dead-on.

So if that is right ,
How would one know if turrents are for mils or Moa?

Mils are used in the reticle.
MOA adjustments are found on most target turrets.
There is a scope manufacturer out there who is building a reticle based on MOA and can't recall who it is.

I'm Looking at a Leupold 6.5X20 . Didn't see any options on turrnets, only recticles.

If you looked at Model #57175, it does not have target turrets.
You need to look at a model like #57180. That has the Varmint Hunter reticle though.
My 6.5-20 is an earlier Long Range/Target Series and looks just like the #57189.
The latest improved version of that scope is now being made under this model number #54680 which is a Mark 4 series.
If I end up getting a 243Win, I will probably buy this model #60080 which has the TMR.
There is also a Mark 4 series scope that has a target turret which moves 1" for each click. Those are called M3 turrets and is more of a specialized scope for the military.

Overall the MOA system seems faster than using the the mil dot, but has the extra expense of the rangefinder.

Remember, you are using the mildots for hold-overs/unders, not ranging.
MOA is slower actually.
Say we are hunting together with the same gun, scope and cartridge combo and we see a coyote at 350 yards.
If I know that my first mildot down the reticle from the center will get me a hit on that dog at 350 yards, by the time you have dialed in your 3.5 MOA (14 clicks), I've already killed the coyote.
(info for the MOA and Mils needed were taken from the drop chart posted below)

If you are looking at a 6.5-20 Leupold, go with the long range series that has the focus adjustment on the left side and invest in the tactical milling reticle right away. You may think its a bit overkill now but you will grow into it later. Any of the top of the line Leupolds are pricey but I feel that glass should cost at least as much as your rifle if not more.
You could try the varmint hunter reticle but that reticle is based on each animal being the same height every time you try to range it. Not sure about that concept.

I've included a photo of a drop chart that is printed on an Avery label and then stuck to the inside of my Butler Creek eyepiece scope cover. It gives a quick reference to my drops.