On the way home today I smelled a plastic burning smell similar to insulation burning.

I narrowed it down to the passenger side fan ...with the car off the fan does not spin freely ...I have to probably put about 5-10 pounds of force to move it...

The question is - Can I buy just one single fan and replace it...or do I have to buy the WHOLE fan shroud assembly with two new fans?
And if I can buy just one, is there a difference between the passenger side and Driver side fans?
and lastly does someone know the part number?

 

it appears that the fan motors are the same - whew and it is part number
21487-CD00A for a 2005 350z

 

It looks like you answered your own question, but yes - the motors are the same on both sides and you can just replace that single fan motor instead of an entire assembly.

Here's a good write-up in case you haven't found it: https://my350z.com/forum/engine-driv...-from-top.html