I don't know weather to replace my ipod, or just buy a new iphone 7plus and store my music on that. I am worried that the battery life won't be good enough on the iphe 7+ as i listen to my music for most of the day. Also will having the Bluetooth on, will that use-up more battery power, as am planning to go wireless in the headphone department.
Go with the iPhone. It has up to 60hrs of audio listening time, which should satisfy your needs of listening to music most of the day. The latest iPod (iPod touch 6th gen) has fairly outdated specs (iPhone 6 grade specs), and to be fair isn't that great. The iPhone 7 Plus allows you to do quite a few things that the iPod touch doesn't, and the iPhone 7 Plus is really, really fast and snappy.
So, IMO, go with the iPhone 7 Plus. It's the better choice.
It just keeps your phone charged until the battery in the case is dead. I don't recommend it as it adds quite a bit of bulk to the phone, and the iPhone 7 Plus has a 2900 mAh battery, so it should last you an entire day before needing to charge it at night.
I say the iPhone 7 Plus. The battery is great on the Plus, even with heavy use you should be able to get a full day of use from it (12-16 hours worth), and with higher available memory capacity
The one exception would have to do with what headphones/earbuds you'll use, since the iPhone 7 doesn't have a headphone jack. Since you mentioned Bluetooth, it doesn't sound like this would be an issue -- but the lack of a headphone jack (even though you can use the adapter and the lightning port) does bother some people.
I have 50gb of music on my iphone and listen to it much of the workday. I have no problems going a full day on battery. If I am pressed for juice I give it a little charge on the ride home, but that is very rare. I say get the 7+.