Group MMS is the one frustration I have with Android devices and my carrier but I'm not interested in switching carriers because I like the coverage that I get in my area and the data plan that I have. I'm on AT&T which seems to have compatibility issues with Samsung phones that have advanced messaging enabled on another carrier *cough*tmobile*cough*. I bring this up because you're coming from a iPhone with which messaging just plain and simple works no matter what, but with Android there's a chance you'll have issues. If you're on AT&T and frequently group text with T-Mobile Samsung users just be prepared that you may not see their messages in group chat.
As for transfer of messages i have no idea, I doubt it's possible but I am not sure.
Update:. As you can see below other carriers don't necessarily have issues with group messaging. My recommendation is to be mindful of the potential issue if you are on AT&T but to not let it sway you away from purchase if you are on a different carrier. I have read that on AT&T the issue can be mitigated by having users that you have trouble with change settings on their end. I have not had success with this most likely because the user I have issues with either couldn't figure out what I was asking her to do or refused to do it.
To answer your other questions, if you already have some Google Drive use occuring on the iPhone side the only reason you wouldn't be able to use the data on Android side is if the application that's storing its data on Drive doesn't exist on Android. Check the play store for the application (
https://forums.androidcentral.com/e.../https://play.google.com/store&token=mT6nhmGf) to see if you can find the note taking app you need.
File management on Android is pretty good. There are a ton of different file explorer applications to choose from on the Play Store. My recommendation is Solid Explorer but there are many others like it. If you do buy an Android device and install a file manager app be sure to test out the share menu from other applications to see if the file manager shows up (most do, some don't) so that you can share a file/attachment to any folder you wish on your device using the file manager. Handy feature for sure.