You can get different weight yarns from drop spindling, as well as plying.
I think a spinning wheel will give you a more consistent yarn, due to the fact that with a drop spindle you have to keep stopping and starting, and on a wheel, you just spin and spin and spin. However, your experience level will factor greatly in how consistently you spin with either tool.
My Ashford Traditional is about 2.5 feet wide, 3 feet tall and 1 foot deep (approx). Here it is in front of a standard sized fireplace.
I also have another that is almost twice the size of my Ashford, but it is a custom hand built wheel. Other wheels can be smaller, taller, narrower etc. Most commercial wheels are pretty small and don't take up much room. Plus they make great conversation pieces so leaving them out in the open is a plus!
Wheels are generally all inclusive and some even come with a supply of wool to get you started.
No, all drop spindles are not created equal, although all of them, even the most primitive can be used to create yarn. They come in different weights, some are top whorl, some are bottom whorl, some are wood, carved and plain, some are plastic, or made from stone. Spindles can be a collectible so there are many different types. Usually the more expensive ones are better quality, like they'll spin better and more evenly without wobbling. Or they're just more expensive because they're beautiful. But for the most part, any spindle will do. Even one made with a dowel and CDs.
A drop spindle is a good place to start, since they can be so cheap, but you should be aware that spinning on a spindle is much different than spinning on a wheel. A spinner may love one and not the other and vice versa.
Hope this helps! If you have more questions, just ask!