Win 8 is about to be replaced by win 10 on phones because MS say it plays more nicely cross-platform with "proper" windows 10 which sort of implies that the current win8 version doesn't. No idea if that's right or not, just something I read.
I've always had Samsung phones and found them to be well made and pretty robust and reliable, even surviving being dropped and submerged in water- don't ask, lol !
As you are buying the phone carphone warehouse is worth checking out the deals on all networks as although it might say X amount on o2 and Y amount on EE, in reality unless it's 3 network, the phones are not locked to any network, so just get the one that is cheapest and it will work with your sim. Someone who works for them told me this. Basically it's so they don't have to keep stocks of each phone for each network even though they are advertised as such. 3 network is the exception here - they are usually more heavily discounted and therefore locked. I bought mine "on o2" as it was £30 cheaper than my network, but it works fine. Not sure if this applies to Currys/PCworld stock or not seeing as they are now the same company.
You might also want to check out giffgaff for your sim. It uses the o2 network, the sim card is free and depending how / how much you use it, the bundles are pretty good value. Disclaimer: I don't use it myself as I predominantly use my phone as a message service and therefore go with one that has free voicemail, whereas giffgaff does charge for message retrieval ( or at least it did last time I looked).
Don't know much about apps - I'm not one of those that spends his life glued to the phone, so beyond a few novelty items when I first got it that were soon forgotten I've never bothered.