A Galaxy Tab with gps and Navionics does for me - been using it for several years now. (I'm not an iPad fan). A friend of mine uses his tablet in preference to his (expensive) Raymarine plotter...
I have an old Hudl that lives in a waterproof map bag for hikers. It has two disadvantages, battery life is poor - it needs to be plugged in if it's being used for any length of time, and it's hard (nigh on impossible) to read in bright sun. When the time comes, screen brightness will be a determining factor in the choice of a replacement; the battery life is far less of an issue as I have several USB charging sockets it can be plugged into.
I have a 12" Samsung Galaxy Tab Pro S, which has pretty good daylight viewability, although it's not that cheap. For cheap i'd suggest a 10" Lenovo, look on Ebay, Argos usually have some factory refurbs on there for about £80, not as good in bright Sunlight as the Samsung, but much better battery life.