Best Wheels

I just put 20" wheels/tires on my 2004 Pontiac Grand Prix GT. I want to lower the car now and would like suggestions of possible damage to car, engine, suspension or any part of car. Any ideas for price and product to lower car safely would be helpful as well. thank you.
The regular size that I bought the car with was 16" tires. Can I get information on lowering the car without issues down the road. I would rather do all work at once to prevent problems later.

I have a cadillac cts that comes stock with 16inch alloy. I would like to upgrade to some 18inch black wheels but a friend of mines told me that i will lose a certain amount of horsepower/torque for every inch i go above the factory specs. A friend of mines had a 2000 pontiac grand prix that came with 16inch alloys and when he upgraded to 18inch wheels his car did seem a bit slower. What do you think? Do you think adding bigger wheels and tires to a car slow it down? If so, by how much?

Whenever I park my car, I like to turn the wheels because for one, I think it looks cool (especially when my car is clean) and also because I heard it makes it harder for crooks to steal the wheels or the car itself. But I wonder could this damage my car over time?

I drive a 1990 Pontiac Bonneville in good shape. Thanks!
My model does have power steering.