bumping up to 18s
 

what would it take to bump up to 18s other than wheels and tires? how would it affect wheel gap,ride height, and ride quality?

If you want 18s that's all you have to do is wheels and tires. It's not necessary to modify anything else on the vehicle.

Highly dependent on what wheels and what width and what offset ... I would check the wheel posting and see what you like as a style, then start asking the more specific questions. Some wheel and tire combos will require a lowered car and possible camber adjustments ... which means suspension will need to be addressed.

The stock tire is about 24.6 inches in height. How do we get that figure (short of going out to the car with a tape measure)?

The tire size tells us everything we need to know. The stock size is 215/45-17. Lets break this down:

215 is the section width in millimeters. Dividing this by 25.4 we get 8.46 inches

45 the Aspect Ratio and is the height of the tire as a percentage of the width. So 8.46 inches multiplied by 45% equals 3.81 inches. If viewing the wheel/tire from the top of the tire to the bottom, the 3.81 height occurs twice, once above the rim and once below, so the total height is 7.62 inches (3.81 x 2).

If we combine the tire height to the rim size we get 24.62 (7.62 from above plus 17).

So if you wanted to fit an 18 inch rim and wanted to keep the stock ride height you could back into the correct size (width and aspect ratio) tire. First you would want to choose a width of the tire. For argument sake, lets assume that you choose a 235 wide tire. 235 divided by 25.4 equals 9.25 inches. We know that our "allowed" height is 24.6 inches and we have an 18 inch rim. So if I subtract 18 from 24.6 I'm left with 6.6 inches. This is my allowance for Aspect ratio. If I divide 6.6 by 18.5 (9.25 inches above x 2) I get 36%. This is close enough to a commercially available 35 Aspect Ratio.

So the perfect fit 18 rim would use 235/35-18 tire.

Math is our friend. :)

would it be possible to switch to 18s and lower the ride height without coilovers or springs?

For most setups yes but do you really wanna see that wheel gap in your car? if you don't mind that then you don't need to lower the car if you do mind then get some springs if you're not into tracking your car and into suspension tuning.

Essentially this guy:
is saying that what truly matters is the TIRE not the wheel. The diameter of a wheel is likely the least important consideration when it comes to fitment.