Yes, you can download the maps via wifi. That's how I do it since I also don't have an unlimited data plan.
The best scale for hiking is the 1:24,000. This equates to the 7.5-minute USGS Quadrangle maps.
After you download the map tiles, you may have to go to Options>Data Manager>Maps>Mercator Lat Lon and open the tile from there to get it to show on the screen. You may not need to do this, and should only have to do it the one time when you first start using the app.
My only negative about this program is that the map files act like images as you zoom in and out, i.e., you start to loose resolution on the screen once you zoom in past a certain point, unlike, say, Ovi Maps, where you can zoom in as close as you want and the lines on the screen stay at the same resolution. This is only a minor issue since you can see enough of the topographic detail of the spot you are without having to zoom in too much.
