Question [Q] K900 row rom and how which custom rom should i use!

Hello and sorry if this thread is in the wrong place! (if it is please move it to the correct one).

I recently bought online from aliexpress an original k900 phone. Since the phone is not sold officially in Greece and i wanted it to have the greek language (at least be able to write in Greek) the seller told me he would put a rom that is universal and has all the languages possible and so he did
Before i ask tell you my issue and ask any questions i will provide you with the software and rom characteristic of the phone as it came from the seller.

Hardware Version: H-3-01
Software version number :S-3-01
Android version: 4.2.1
Baseband Version: CLV_6360_MODEM_01.1318.F
Internal version number: K900_ROW_LA_1_S_2_009_0035_130822

Also it has superuser installed with the following
superuser v3.1.3(46)
su binary v3.1.1(17)

From what i can understand is that its one of the original stock version lenovo provides but with international languages. (maybe i am mistaken)

1st question

when i go to settings-->all settings-->about device-->system updates its tells me that i can upgrade to K900_ROW_LA_1_S_2_009_0069_130922 (3.78MB)
The question is if i press update does anyone know and is sure what will happen? will it go back to the original asian market languages that the phone is intended to be sold or will it be ok? According to the seller he told me that if i press update it will lose all the languages and it will mess up the phone. Can anyone help me on this, if i press update will it mess up the phone? is there going to be another update after that version? or is the version mentioned above the last update available?

2nd question

From day one i have noticed two issues with the phone that from my experience both have to do with the current rom. The 1st is that i cant disable the stock messaging app in order to use go sms pro for 160 characters support since i write in greek. And since it cant be disabled from nowhere and even though i have set the go sms pro as the default i receive one sms 4 times, 2 in the stock sms app and 2 in the go sms pro app and if i delete one the messages from one app it deletes the sms from the other app automatically. The 2nd issue has to do with the proximity sensor and the screen turning off all of sudden or way too fast and from what i observed it does it only when using the stock rom and not in other 3rd party apps.

The question now is can someone please tell me what is the best and most stable custom rom that has multi language and the instructions on how to root.I have rooted many phones before mostly samsungs but they all had removable sd cards and it was clear. I am kind of an experienced user and i know my ways around a phone, but this is my first phone that has all non-removable parts and i dont want to brick it or get it in a infinite loop since there is no support for these phone in my country and i would have to send it back to china for repair or replacement

Thank you for your time and forgive me for asking stupid questions or putting this thread in the wrong place!