If you are considering the iPad as a laptop/desktop replacement, you will probably want the larger iPad Air. Don't under buy, thinking you can get by on 16 GB of RAM. You're probably going to be using this device for a couple years, so leave yourself some room to grow.
If you think you are going to carry it around a lot then give the wi-fi + cellular models some serious consideration. It's not just about having an internet connection everywhere. That GPS is really handy and only comes with the cellular models.
Whether the iPad will do everything you are used to doing on the laptop is going to depend on what you did on the laptop. There are some tasks that are difficult or impossible on the iPad. You'll also spend quite a bit of time figuring out how to do things differently. iPad's do not have a general purpose file structure like a desktop. Instead of having a bunch of files and then opening in something, You'll find yourself picking apps to keep your files in.
Basically, if you can do it on your iPhone you can do it on the iPad, only with a larger, easier to use, interface.
This will change to some extent with iOS 8, due out this fall; but it still won't be exactly like a laptop or desktop.
