It is best to consider the iPad as a sealed unit. Whatever model you purchase, there is no upgrade path if you decide at a later date you need more memory. It is not possible to read and write to external memory cards from an iPad, at least not in any meaningfully useful manner.
So buy one with as much memory as you can afford. The models with cellular phone network connectivity have decent GPS. The mobile network is used to speed up geolocation and lock on to GPS position more rapidly.
Get the highest spec and most memory you can afford and you are unlikely to be disappointed. Apple devices have a decent lifespan and will cope with several generations of updates before they need to be updated.