Downloading Gaia maps now; estimate state of Washington, Gaia base map, MVUM maps and USGS topo maps at zoom 15 will take 25-30 gigs.
Be careful! The non-cellular iPad does not have a built in GPS! You will need the cellular model to get a built in GPS! If you have an iPhone you can tether an iPad to it and get GPS from the iPhone but it does not seem to update well and eats the iPhone battery really quickly (and iPhone will get hot). I have tried this with old iPad2 and it just did not work well enough to be usable. Between battery draining while being plugged in, slow GPS updates, iPhone heat and the slow iPad2 it was not worth it. Recently got iPad Pro 11 with 256Gb and cellular and that seems to work well with enough space for extra maps and music.