We use the Midwest metal ones. Height, and door or no door depends on how easily you can step over teh sides. 24" is high enough for most puppies, and it's what most of our puppy buyers use. If you can step right over the side, I wouldn't bother with the door.
The pup needs some potty system inside the pen, but can also learn to use the bells when she's loose. A pad will probably just be something to shred unless it's under a grate.
Expens are okay for outside, but they really need more room than they provide to run around when they are outside. It's more to limit the available space, than to provide a place to play. In fact, we always start with the pen folded in to 2 x 4 feet. The idea is acheive success in an area, and gradually increase the area. You have to start as small as is necessary for it to work. This is why we start them at 3 weeks old while they are still in the whelping box. It's all about getting the pup into the right habit. There is zero reasoning about it at an early age, and might not be until they are a year and a half.