1Mb/s should be ample client side.
The number of mods on a server does not really change the required bandwidth by much. It does depend on the mod, but since the mods are serverside they will mostly affect the server resources. Your internet connection will only be sent extra information in some scenarios. If you are lagging it is most likely because of the ping or the resources on your own PC. (Some people will disagree here, I am ok with that).
This is a really hard question to answer. Data is used when there is an event that fires on either the client side or the server side and then information needs to be sent to inform the server or client of the event (this is a really simple explanation of what happens.). The server side is the one the has an effect on how much data will be used. It will depend on how many players are on the server, what sort of mods are working and how often certain things, such as holograms, are updated.
My server, which is a small Australian server, has used 4.64GB this month so far (4 and a half Days). We have about 10 regulars with about 10-15 others that come by every now and again.
Hope this helps in any way.
This does help, I will look into server types and pricing. I need to check on my side for network usage after an hour of playing.