No I'm not kidding. You can usually arrange a 2 week trial if you go to a physical store, to test the transfer rates at the places you're going to use it at.

The coverage maps only tell you what signal strength to expect, and aren't that accurate or up to date (when the operators build 1000 new transmitters per year and update map twice a year, it's always out of date). Besides, signal strength doesn't translate into transfer speed. The speed will depend on how many users in a cell, how heavy they use the service, the amount of channels allocated to that cell, and the size of that cell's uplink. The operators consider that information, as well as the actual location of their base stations, proprietary information and wont tell you. The only way is to test.

There are no transfer limits or extra charges. Some operators explicitly forbid p2p traffic, and all of them prioritize voice over data.