The PRS-MCPS license is easy to get and covers the music copyright on behalf of music writers, composers and publishers - this is a blanket license and you pay a flat fee.
The PPL one is a bit more of a headache and covers the recording copyright, I believe you have to track how many listeners hear each track and report it all although I've never dealt with them. I know of a radio station that pays the PRS one but gets permission from labels for the recordings as they are genre specific and have good relations with labels.
Yep! You need to report how many listeners you have, how long they've listened for, and you generally need to Geo-Lock your station to the UK only (if you're from the UK that is)
The PPL billing works like your electric company. They'll ask for your logs every few months, and the months in between, they estimate from your previous results etc. Or at least, that's how it works at the station I work for, but that's commercial FM, so im not sure if it's a bit different for Internet Radio.