New Smartphone advice needed

Hello all,

Short-term lurker and first-time poster here. Recently I was sitting in a pub and saw an app for Murphy's that gives you free pints when it rains and figured that now would be a good time to upgrade my phone, so I've spent the past week and a bit reading threads here and in other places looking for recommendations and advice about what phone to buy.

I had a cheap thing before (Vodafone Chat, I think) but it would hang up during calls and restart itself for no reason and generally fight with me whenever I wanted to do something other than use Facebook so in the end I let it fight with a wall. It lost.

Having done a bit of research I'm leaning toward an Android. I did look at the Lumia 800 but I'm not sold on Windows, especially with Google claiming they'll pull support for the Windows OS (although you can apparently run Android on a Lumia. Is that true?). Basically, I want a relatively cheap phone that will allow me to get used to Android for a while before I commit to buying a fancier phone when I've become more accustomed to it, or sell on if it dislike the OS (I've become interested in modding/rooting as a result of all the reading I've done and I reckon I'd rather risk botching a cheap phone than a pricey one).

I dislike Apple so I won't give their stuff any further consideration.

I'm on Vodafone PAYG and I'm comfortable with that as many of my family/friends are on it and its fine for getting free calls/texts/internet on it. I did see that Lycamobile offer with unlimited things for between €10 and €30 a month but I'm not sure about them in case something goes very wrong and I need a lot of help fixing a phone. I could probably be persuaded otherwise though.

As for phones, I did look at the Huawei g330 on Emobile for €150 (I'm guessing that goes to about €170 with unlocking) but I did see the Sony Xperia P for sale in England for €190 or so, and also looked at the Jiayu G series in China but I'm not so sure about that, tbh. I'll be sticking with a budget of €200 to get me started even though I know that I'm essentially limited in what I can get. Some basic stuff I'd like to get would be a solid camera (5mp minimum) and something that can handle me web browsing a lot e.g. taking pics and uploading them to Facebook/blogger/photobucket without slowing down too much. I was worried about space but with things like dropbox and SD cards I shouldn't have a problem.

I have a few questions though.

1. If I buy a phone from the UK or China unlocked, does that mean I can use an Irish Sim card in it without any further work?
2. Speaking of Sim cards, the Xperia P says that its microSim only - can I get Vodafone to swap my existing Sim over for free or do I have to pay a one-off fee or monthly fee to do so?
3. What difference will it make to a phone if I run it on Ice Cream Sandwich or Jellybean?
4. Is there that much of a difference between a single and a dual-core processor in light of what I'm seeking to do with the phone as stated above?

Thanks for reading.