The problem of the L2J is the java, you use a virtual machine and you don't hit directly the processor native instruction like when you compile in C++.It mean more ressources.
For 1000 Players online without any lagg you would need that :
-Quad core xeon 3220 or any quad core CPU , maybe dual core might be enough , I only tested with quad.
-8GB of DDR or more.
-SAS 15k rpm hard drive
-100mb/s bandwith
And anyway , 1000 player is hard to hit exept if you plane to make a low rate , but against L2P that will open lol ...
If you have no experiences in the Lineage 2 Server , I recommand you to make a L2J and since the topic was about the security , L2J is under GNU license so it's more legal than L2OFF ^.^