Next, software. The iPad built-in maps functions all require an active internet connection of some sort (3G or Wifi) since they download satellite and map data "on the fly". In contrast, applications such as SkyDemon, PocketFMS, Air Nav Pro and even TomTom download the maps that you think you need in advance, and do not need to download anything "on the fly". Obviously within the app you need to specify the areas where you're going to fly, and for which the maps and airspace data should be kept up to date. The more areas you activate, the more data needs to be downloaded and stored on the iPad. And in case of some products, the more money you need to pay.
If you do have an internet connection while flying, somehow, then at least SkyDemon is able to download things like weather "on the fly", which may be useful on long flights. I don't know if other products can do this too.
FYI PocketFMS and EasyVFR by PocketFMS (€69 on Android or iOS) can download weather and advanced weather (72 hour, hour by hour forecasts from Top Meteo...no additional cost) on the fly, along with EuroControl NOTAMs.
As others have said there are two types there are two types of iPad; wifi and 3G. Only the 3G has built in GPS and it is NOT dependent on phone receiption. If a phone receiption is available it can speed up the time to first fix, but it is not required. The wifi version does not have GPS, but it can get a reasonable fix from phone masts while on the ground. It won't be able to in the air, becuase it doesn't have a GPS.
You also mention about about driving maps. As others have said these can be different. Apple's built in maps and Googles free maps both use 3G receiption to pull down map tiles as needed, so if you don't have a 3G connection then you won't have any maps. Of course when you are away on holidays you are roaming and this is when 3G tends to be expensive!
As BackPacker says, teathering to your smartphone is the way to go when at home. When away I'm afraid the answer is probably different for each country and probably involves getting a local sim.