Help me choose a new phone / some Android questions

Hey everyone!

I am currently using an iPhone 4 and since it's starting to show its age, I think it's time for me to upgrade. The last Android phone I used was a Samsung Galaxy SII and it had Android 2.3.something, I sold it just a few weeks before it got an official ICS update, so I'm not very up-to-date on Android. This is why I'm writing this post.

So first I need help picking a phone. I live in Europe and I'm a student, so I'll be getting an unlocked phone and of course it won't be the latest and greatest (wouldn't have any problems choosing if money wasn't a concern). So the price range I am looking at is 300-400€ and for this I could get a Galaxy SIII, but before running off to the store, maybe you guys can recommend a better option? Since I'm on the iPhone right now, I think it's really important that my phone should support future Android updates. The build quality too is important to me (too bad I can't afford the HTC One) and since I like to tinker (I hope it's the appropriate word) with the software, I want to be able to root it. So any recommendations (and reasons why)?

And secondly, I have a few questions about Android in general.

1. How is the app department doing? Last time I was on Android, it was a huge mess, stuff wasn't optimized and you could have a mobile app blown to full screen if you had a 10" tablet. I also remember that the official Twitter client lagged a lot. Has this, and the choice of apps improved a lot? I've heard and read that the situation is better, but I'd like to hear some real opinions.

2. Is the Galaxy SIV's S-stuff (not that I really care, but just wondering) a SGS IV exclusive feature or can this stuff be ported to other phones? i.e. the Galaxy SIII has this Smart Stay or something, which turns off the screen if you're not looking and the SGS IV has this Smart Scroll or something, which scrolls the page once you reach the end. So in both phones there's a sensor watching you and I'm just interested if this can be ported or something?

3. How is the battery life of current Android phones? I'd like to be able to keep wifi/data/GPS/notifications on at all times, use my phone for calls/messages/apps/internet and be able to survive on one charge at least from early morning to late evening, preferably for two full days.

4. Is there something like Cydia on Android? I know I can get custom launchers and that kind of stuff from Google Play Store, but I want to know if there is any marketplace or something from where I can install iOS-like small tweaks, i.e. be able to move status bar icons around or have something like Zeppelin, Zephyr, Emblem, Auxo, Velox, f.lux, etc. I know that Android is very customizable, but I've never heard of tweaks like they are on iOS.

5. How could I move my stuff from my iPhone to an Android phone? I am currently using iCloud for all my contacts, so what is the best way to move them all to Google? I'd also like to get my notes.

6. How can I make my iPad and an Android phone work together? Can you recommend any apps that are present on both platforms? Like task managers/notes, etc.

7. How are the phones holding up after they have been filled with content and used for several months? Will I experience any slowdowns? How to avoid them?

i think these are all the questions I have for now. If you think that there's definitely something else I should know about Android, please let me know!