T-Mobile is discontinuing their support for the HTC 10. I don't think any software updates will come if you have the T-Mobile variant, unfortunately. There's been a lot of rumor mill on Reddit and XDA about this, most of it pointing to low-sales numbers over all the variants of the S7 that Samsung launched at once.
In all honesty, if you can't afford the phone outright as it is right now, you just can't afford it yet. Save your pennies, buy it when you have enough scratch on hand.
I use the same thought process for my other hobbies. Be it my old '98 Z/28, or my 1/8th scale RC fleet. If I can't afford it all right now, I can't afford it yet. I don't like being pinged monthly for payments on things if I can avoid it.