Setup new phone from iCloud backup

Long time 5s user but have ordered a new 6 plus device. am planning to restore iCloud backup from my current 5s to maintain settings when it arrives. I know that you cannot restore a ba clip from a later iOS to a device running an earlier one. So the backup from my current 5s is based on iOS 8.4. There is a chance that the new phone may have older iOS on it such as 8.2 etc. when I switch on 6 plus for first time and login to cloud account and select my backup will it auto detect if an earlier iOS is on it? And then auto download the latest? I know I could go in and setup as a new device, join wifi and then update OTA but it would be slightly time consuming as afterwards I would have to then erase all settings to then get at my backup if that makes sense?

finally when I have done cloud restores before the slightly frustrating saga of the device wanting to re download all my purchased music occurs. At nearly a 1000 Songs this takes a long long time - is there a way to avoid this happening as I'd rather get the phone and my apps functioning and then re download songs at my own leisure a few at a time over the next few days. Since the last time I did it I have started using iTunes Match (not apple music) - I hope maybe that being enabled by default may suppress the restore trying to re download all the music bought from the store. I sincerely hope it doesn't try and do all my match music which includes cd upload/matched songs as that will be more like 5000!!! It just means the phone has to be left alone for a long long long time when the music downloads.

cheers in advance,

wayne