@GANESH KACHAM Go for a used one... I know someone who deals in enterprise grade systems all over india.
http://www.zacocomputer.com/
Company: Zaco Computers
Contact: Zameer Kazi
Email:
Zameer@zacocomputer.com
He is owner of the firm, one of the largest in India that deals in Second hand Server hardware.
The server you've listed will work well for a Domain Controller (DC) & File Server nothing more, but still at this point you need to think about data backups & redundancy. not having a plan here will kill your business.
If you are planning on doing Remote desktops etc. you need more CPU & RAM, a Dual Socket CPU with atleast 16Gb Memory.
from what i am estimating you need about 300+ mb per user on idle use add about a few more for browser,Tally & Office etc. it should be around 500mb per user/ session so thats 10gb + 2gb for windows.
Also its not recommend to have a Domain controller & a remote desktop server on the same system so you'll need to create a vm on the DC so 16 gb should be your bare minimum.
Use a regular 24 port Switch, you don't need anything fancy like a managed switch. with what you've listed out in the OT your files ain't going to be that big to cause any sort of network bottleneck. It will help though if your server has dual LAN's then you can do NIC Teaming.
As for what brand, you can take your pick, you will have people siding one brand than the other always. its a matter of preference, I've used Dell in my entire rack.
@vivek.krishnan Thats the problem with IT guys in India (me being one)... my previous employer where i used to work at an non-IT position, their IT team used to come to me to ask for my assistance in troubleshooting issue & talking advice...& back then though i wasn't even a certified guy, just knowledgeable. although in all honesty, this is a field where everyone will not always know everything.
P.S. on a separate note, If you do need some help getting things setup & working in, send me a message. I am an freelancer, I do these things daily.