Hi Mullionaire and welcome to NICO Club!
Before reducing rim size to 15", you might want to make sure that you can clear your brake calipers in the front and rear. It's 1 inch all around, so just grab a measuring tape and make sure you have at least 1 inch of clearance (at least!). Pay attention to offset when looking for the steelies. You'll want to center your new wheels in the same location as the center of your old wheels or possibly match the outer face as opposed to matching the inner face of the tires.
For a tire size, I'd try 195/65-15 since that is pretty comparable to your OEM size.
Here's a link to a cool calculator with graphics that will help you visualize the effects of making changes in numbers:
https://www.wheel-size.com/calc/