1)Dont think you can remove the 3G icon, but i use wktask to cover it up and other unwanted icons(battery, volume, etc) its adjustable.
2)If you dont use the dataplan i suggest using Modacco Nodata to disable gprs and medianet. Its a good way to make sure you dont accidently use their service. can save you a loottt of money!
3)As for automatically set wifi, dont think there is one. Its just best to leave it on. In my opinion wifi doesnt consume that much battery when its on(aside from just being used to surf, etc. and it shuts off when phone is sleeping). But then again, you have to consider the issues of wifi availability, and configuration settings(wep, etc).