Currently, I use MobileMe to purchase my apps. If I want to update my apps, I need to log into my mobileme account to check for updates. So what happens if I cancel my mobileme subscription one day, how will I be able to do so?
You are a bit confused. You are not logging into MobileMe to purchase apps. You are simply using the same login ID as you are using with MobileMe to login to the iTunes store. When you cancel MobileMe, the same iTunes store ID will still work, you will just not be able to log in to MobileMe any longer.
You should then update your email address so that you will continue to get store receipts emailed to you.