I had the Note 3 prior to my Note 4 and I loved it. I upgraded to the Note 4 and gave my Note 3 to a friend who needed a phone on my phone plan and she enjoyed it until upgrading to the S7 Edge. I now have the Note 3 back in my possession and am amazed at how the Lollipop update increased battery life. Using it now for my goddaughter to play games on.
You can expect a totally locked boot loader and to be the last in the world to get an update. Get the international version or better yet change carriers. Now at this time all US carriers except AT&T have released updates to Marshmellow. Samsung although I'm sure they have the signing keys refuse to do anything and AT&T will not release any information by policy until after they release the patch to avoid confusion. Apparently they don't realize leaving all the Note 5 users totally in the dark causes confusion.
AT&T models are inferior to all others models of the exact same device. I highly recommend Samsung stops partnering with them to release custom devices, and if you must use AT&T bring your own device.