Mine is on ATT, no contract. I use the 250 MB plan, always use it at least occasionally. It is something like $10 or $15. Any month I'm using more I can upgrade the daya plan that month. Nobody said anything about a new SIM, don't even think it is removable without going into the shop.
You can alway get a phone or cellular device to provide WiFi, but most I've looked at cost more on an"in case I need it" basis but less if you are actually using a lot of data. I typically go through 40 - 100 MB of my 120, places where I wan't to connect and don't have WIFi available. I don't do high volume stuff like software updates, downloading or streaming media, while on cellular.