Well, to be honest, there are so many variables which can effect lag its best just to try out and make sure you ask your friends if its lagging. However... for your questions -
1) Yes, and Blockland has to be up if non-dedi, if dedi, only the console has to be up
2) If low bandwith you will feel internet lag, and if low memory yes, as much as normal blockland I believe.
3) Maybe, Connection strength, memory amount, location, other programmes etc.
Honestly, you seem like you would be an intelligent host and I hope you decide to host. I can't forsee an immediate problem if you try.