Hi Daniel,
The Pack n' Play isn't really a special request. Disney has many, many thousands of these on property. If you need one, they'll get you one.
The first floor near buses thing is a request. In my experience, you should ask for things like this early and often. I'd call the resort now and ask that this information be put into your file. Then call the resort back about five days before your trip and ask again. My friend Bret does a great job of describing
how to make an effective room request. You may also want to visit the independent website blog.touringplans.com and search there for an article called "The Art of the Disney World Room Request."
As for on-line check in, if you've taken the steps above, you should be fine checking in online. If possible, make a note about your room location preference there as well. (Sometimes there is a screen that allows you to do this, sometimes not.)
Good luck with your planning. Please let us know what else we can do to help.