Prabhat,
1. For vendors in India, I am sorry, but I do not recommend this as I stay neutral so that I can help anyone.
2. The you can connect the phones directly to the PBX ports, that will work fine.
3. Yes you can put both units on internal LAN, but you will need 2 static public IP addresses as you need to map 1 static public IP to each unit that will send or receive calls from public IP.
If you need more help, you may contact me at mike_voip@hotmail.com
Mike