You need to get wheels to fit your Escape (correct bolt pattern, so they can be attached), and then tires to fit the wheels you've purchased. The two things are related, but not the same.
The OEM tires on the Escape's 18" wheels are size 235/50R18. The first number, 235, is the width of the tire's tread, in millimeters. The second number, 50, is the aspect ratio, i.e., the height of the sidewall divided by the tire's width. The "50" actually means 50% -- the tire is half as "tall" (at the sidewall) as it is wide. The "R" just means radial; sometimes this is left out, because most tires are radials these days. If the tire were speed-rated, there would be another letter in front of the R, such as H, V, or Z. (Sometimes the speed rating is also shown elsewhere on the tire.) Finally, the "18" refers to the diameter in inches of the tire's opening, and therefore of the wheels it fits.
However, the wheels you buy may not be the exact same size as the OEM 18" wheels, though matching them would be a prudent thing to do. You need to fit the wheels you do get.
If you're going to store the wheels/tires as units until you need to put them on again, rather than taking the tires off the wheels, I think it makes sense to keep them reasonably well-inflated, but others can chime in on this point.