If you had a legit version '( a cd-rom) then all of this wouldnt be new to you.
The mod that is being discussed is the 'Baddog' Mod by Shaun Fletcher. It can be found on the disk, or here. If my memory serves me right, then the OC mod and the Imperia mod both build further on this mod.
The other features discussed ( internet options & Vizor) are the GSC server where you can play aswell, but you require a password for that ( orbetter said, the serial code found on your manual, which obviously you don't have) and the vizor system is a system where you can watch played games or games in progress. Also to be found trough the GSC server.
About the latter, dont worry, nobody plays ( virtually no one) there anymore, though it is a secure place. You cant cheat there and you surely can't influence the points/ranks as one can in the gamespy server.
Also, never be influenced by a review of the 'gamespies' of this world. None of them is objective. Why would you let someone else make an opinion for you, you gotta make up your own opinion. Did you know that Cossacks is still top 10 of best sold PC games, way ahead of Age of Empires, so it can't be that bad, right?
I dont know if those tournaments mentionned are still played, but there are some fan tournaments around like the East vs West and the LCN ( League Cossackers Net).
Hope that somewhat helps.