[Galaxy S II Q&A, Help & Troubleshooting] Muthzer the One:
1. Sorry no idea here.

2. By rooting your phone you could remove the branding and use any sim card

2b. The most common version of galaxy s2 is the international one gt-I9100. Depending on the year of when it was made there could be a slight difference in processor. Older processor need more voltage than the newer ones and might not be able to overclock as much as the newer ones. But if you don't intend to undervolt your phone this is unimportant for you. All of them work with 1.2ghz. Which processor type you'll get is some kind of luck I guess.

Gt-I9100 aside therealso are gt-I9100P and 9100g. I'm not sure but I think they work with same specs as the international one (1, 2 ghz and 1gb ram) but they use other hardware and thus it's not as easy to find custom roms like for the gt I9100 I think.

Furthermore there also is the galaxy s2 at&t version. If I'm right this the t mobile version of this phone. It also has the same specs like the other models (different hardware too) but the difference is this phone has no physical home button, which for me, looks better somehow.

I'd suggest to take the international one gt I9100 or the at&t version.

3. Official android version of the galaxy s2 models is jelly bean 4.1.2 which also is required by Google search and Google now. I assume it doesn't matter rather you have android 4.1.2 or 5 regarding the voice recognition.

3c. At the moment it's not possible to install Lollipop on a S2 but it may be possible in the future. You'll have to root your phone therefore and flash/install a custom rom based on lollipop.

3d. Like already mentioned earlier you'll need to root your phone in order to get past android 4.1.2. Depending on your phone model (2b.) It can be extremely easy or a bit more difficult. I'm just used to the international version gt I9100 which can be rooted with an app via one tap. That aside there are various step by step tutorials, as the galaxy s2 is a very popular phone.

If you'll root your galaxy s2 I highly recommend to Google emmc brick bug before doing so.

3e. All official android versions can become slow after some time, especially because of not removable, pre installed apps. With root you have the possibility to remove those and make your phone faster. If you intend to stay on official firmware you should update to the latest (4.1.2).

Talking about getting slower with higher android version is a bit difficult here. Theoretically with higher android version the phone will get faster....at least that's what Google says.

However as already said the galaxy s2 is stuck on 4.1.2. The only way to get higher are custom roms and those can be slow or fast no matter on which android version they are based.

For example cyanogenmod 10.1.3 is the fastest firmware I've had till now and it's based on Android 4.2.2. Jelly bean. Next there's for example aicp rom based on 4.4.4 KitKat. This version wasn't very fast for me. Another KitKat rom, NeatROM, however was faster than aicp although it has same android version.

It always depends on your personal phone, because each phone (or each galaxy s2 in your case) reacts a bit different to it's firmware and also on the way the developer made their custom roms.

You'll have to find out there :D