Is 150 MB / month enough for limited use?
Using wifi == not using your data plan.  So that will help reduce data usage from your provider.

So long as you are not streaming (music/video/youtube/etc) or downloading large amounts of data (podcasts, for example) then you should be fine.  Or at least save those activities for when you are on wifi.

Downloading apps: each app can be anywhere from 100kb to 10mb+ in size.  Save these for your wifi unless it is an update you really need then and there.

Weather/maps: shouldn't be too heavy of data.  Same with Google voice.