How to return to stock rom? GAOSP Beta 3?

I received a used handset from ebay (for a fantastic price!) and it has an outdated version of Galaxo ROM on it. I want to return to the latest official Samsung ROM (whichever that is), so that i can then do some comparison benchmarks before moving to the latest GAOSP. Can anyone help me with a couple of questions?

1) How to return to a stock ROM from Galaxo ROM?

2) Where can I obtain the latest ROM and which is it? Intended use is in UK.

3) Does anyone have any idea about when the next GAOSP (full) Beta will be?

PS: I have never done this process, so please humour me and give me detailed instructions

Thanks a lot in advance!

 

This is very helpful indeed! I take it that the firmware released in August is the latest. But there is four of them there. How do I know which to pick? Is Europe supposed to be XEE? And how about Radio/Phone part? Which one of these contains the latest official Radio/Phone part?

 

Thanks a lot, will do. But that is just for Radio/Phone. How about ROM? Which one shall I go with? I noticed there was a XX ROM from August, supposedly the latest. Shall i go with that? Intended use is mainly in UK, but also on occasion Greece, Spain and Netherlands (for vacation )