Performance on official servers starts dropping slightly with around 15 online, it is still perfectly playable up to about 25 when it starts getting pretty bad. By 30, it is truly awful.
The number of players on at prime time varies significantly depending on the server. The most popular servers will hit the 40 cap every day, the smaller servers may rarely go over 20.
Not all server units are equal, and it seems the ones Gportal rents to Funcom for their official servers are basically the bottom of the barrel.
As far as building limits, the rules are posted on the forums, though they are quite vague and players occasionally get admin wiped and suspended without ever understanding what they did wrong.
Abandoned builds decay. If the owner logs in to refresh, then the build stays until they miss their refresh.