Swamp Cooler Conversion to 12 volt DC

Do you want the same performance that you are currently getting ?
You have a 700 watt motor. If you go with less than that, it will cut down on the air flow. That will mean less cooling.
How many hours a day do you run the thing ?
If you run it 4-5 hours, you are going to need about 700 watts of panels (not counting losses).  If you pay 4.50 a watt for panels, you are looking at about $3150 in solar panels. 

Whether you use an inverter or go straight 12V, the 700 watts would be 60 amps on the 12V side (without inefficiencies or losses.)

I don't believe anything on youtube.

I also don't know how you can expect 100 watts to do the work of 700 watts.
My suggestion would be that you do not do anything in your conversion that would make it difficult to revert to the original configuration.

In fact, if I were attempting it, I would build a test cooler first. I would buy some smaller evap pads and mount them in a box with a fountain pump. Mount the fan and duct it into one room. See how it works. Power the thing with a battery or 12V power supply.

If you were to modify your original cooler with the radiator fan, I would guess that you would only get about 10% (or less) of the current cooling capacity.