You ask a lot of questions but I will try to summarise.
Your core issue is the operating system. I see you are not an Apple fanboi, me neither. So it comes down to Android vs Windows Phone, and here Android wins hands down. My smartphone, tablet and laptop are all synced so I can access anything anywhere on any device. Easy to set up and simple to coordinate updates across machines. Now I have a tablet I hardly use my laptop. My phone is capable of being a WiFi hotspot from my mobile data plan so can support my other devices when out of WiFi range. Google Maps is a great resource as are Google Translate, Drive, Chrome etc.
This leaves you with a huge choice of manufacturers to choose from. The latest iMobile models seem great value and are offered by T Mobile in the U.S.A so can't be bad. At the other end of the scale you have things in the Samsung Galaxy range. I love my Samsung Galaxy Note 2 because it is big and suits my fat fingers. And then there are brands like HTC - poor after sales support in my experience - Motorola etc. The only way to select one is to find a retailer with a good choice and decent sales staff. Here I cannot help you, I live in Chiang Mai.
As for your mobile plan, I am sure AIS have many post pay data options. I use Dtac and have no problems with service and billing.
It is worth taking a while to explore the options and a long term view on all these issues. Then you will buy something that will suit you for a while. Good luck.
Sent from my Nexus 7 using Thaivisa Connect Thailand mobile app