There's no option to switch off pre-0.8 downloads, but you can use the trusted peer preference and set it to a node you know runs 0.8. If you want to, you can use riker.plan99.net which is run by me, but don't forget to eventually unset this so you balance the load appropriately.
The app will sync automatically (if you don't change the setting) when plugged in and charging. So, assuming you have wifi at home and you charge at home, the daily sync will be over WiFi. Otherwise it'll only sync when you open the app. So you can avoid it using up your mobile quota by just not opening the app when you're on 3G.
On recent versions of Android, if you go into the data usage screen, you can tick a box that means the app won't be allowed to use background data when it's not got WiFi.
It is for me, so if you see something different let me know and we can investigate.