I don't know much about this problem, so please take my advices with a grain of salt. The best way to do this is to stick with well-known vendors.
-Will the server be as fast as it could be, I mean a quadcore, lots of memory, large amount of hard drives?
Yes, if you are going to use that to maintain multiple clients.
-What OS will I need to install?
It is possible to use Windows, but I am not sure about the license price needed to run Windows on all clients.
I have never set up a thin client system, but if the computers are only used to surf the Internet and create documents then you might want to consider Ubuntu LTSP
. It's open-source, easy to use, free of malware and not that hard to set up.
-After the OS is setup, will there be any configurations to apply or just plug n play?
There would be a little configuration on the client, but most configurations will be on the server.
-Are there any specifications you guys might suggest for the server and the workstations?
The server should be strong enough; it depends on the number of clients and the client platform you want to use. However, the clients won't need much resources to run properly. It just needs a CPU, some RAM and some software to start the connection. Most modern BIOSes can boot from the network, so this is not much of a problem.
If you don't want to do it yourself, there are many thin client vendors available like HP, Wyse... They often provide the software needed to run the clients, so you don't have to do much to get them up and running.
You can also reuse obsolete computers as clients, and save lots of money. Sometimes you will want to downgrade those computers to save power.