You can use a SSD or a Raid Master 0 via 2xSSD's and your server will never use the SSD potetial, since there is no need for that much raw power ( C5 yO not GOD )
A C4 C5 C6 server would never choke if the server specs would be set on a 7.2k rpm raid master 0, even with 5 k online its simply imposible.
Do you really think i base my words on google ? Sorry but i base my words on my experience. Use your SSD's and your gigabit port, before live use a hdd stat monitor, watch how your mighty ssd's max the hdd speeds on 60% (max).
What 100mb cap do you mean ? Most hosting companies mention 100mbps on download/upload, but the bandwidth is unlimited ( not at every company ), if the bandwidth is unlimited a gigabit port is useless. Plus there wont be any 100mb up/down by player base gameplay on a l2 server.
And about internet speeds even on 100 or 1000 mb port net-pipe flooding will be inevitable in any attack attempt. But still a gigabit port is more than welcome.
People will lag no matter if you have a gigabit port or not, it depends on their connection streaming towards your server. Who knows maybe you will use a re-routing proxy and yet there wont be any guarantees to have or not lags.
What you specificy about SSD and Gigabit port is good, no...its great, but what im saying is that it what you got or will get wont be used on its full power.
Good luck tho.