You're doing the right thing by studying floor plans, but you're going to have to go kick the tires in person to really get an idea. That's because there's such a variety in ramp geometries. Some of the ramps are shorter and steeper. You'll definitely want a unit with a "beaver tail", which is where the last few feet of the garage begin to slope down and the hinge of the ramp is lower than the rest of the floor. This makes a big difference in the slope of the ramp.
You'll also find a wide variety of lips around the ramps, some of which will be really tough with a wheelchair. Ditto for the hinges that connect the ramp to the frame.
Good luck with your search.