Hello Mike,
Great! so what you should do is your phone contract or Pay As You Go? Again if it is contract you should talk to T-Mobile about upgrading. If you are able to upgrade this should lower the cost a fair amount. If you are happy to buy outright for you 6 Plus, give the old google a go and find your nearest Apple Retailer and order yourself the 6 Plus of your choice. I would advise to check to make sure your sim card would work in the phone before you buy, usually it will say if the device will work on all networks or only some. Going through T-Mobile they would get you one that would take your T-Mobile Sim.
With regards to the data transfer, as long as you have the USB cables for both devices it should be fine, with it being an android system, you'll have to go into the device and into the folders manually to copy your data. As soon as you plug in the phone it should pick up on your mac straight away and I believe it's under Media. Simply select all the folders you want to transfer, copy them to your Hard Drive it's probably best to import music to your iTunes as well, when you connect your 6 Plus, go through iTunes to sync your data, within iTunes it will give you the option to select what data you want to sync.
If you are signed in on your Mac using the same Apple ID as you are using on the 6 Plus, you should back up your photos and documents to iCloud. When you sign in to that same Apple ID on your iPhone, the data in your iCloud can then be downloaded onto the iPhone.
It may sound complicated but the device pretty much does most of the work, like I said if you get stuck go into your local T-Mobile store or Apple store and they will be able to assist you and show you physically how to do it. If you go through T-Mobile to get the 6 Plus they should be able to do the data transfer for you as well.
Hope this helps
Rob