In the month or two I will be building up a new 650b carbon hardtail frame. I have decided to use a shimano xt m786 group. That said I have a nice m960 xtr crank that I am thinking about using. I know the crank will work with the 10speed drivetrain just fine I am just wondering if the crank will look out of place. My reasoning is this frame is on the extreme upper end of my budget and saving a couple of hundred of dollars on a crank would make the new wheelset a lot more affordable.

Keep the XTR crank (it's 9 speed, but will run a 10 speed chain fine) till the chainrings are worn, then replace. The XTR won't look too out of place, unless you intend to run 1x10 or 2X10, then it's no good. The major issue with the crank, and the reason for dumping when worn, is the chainrings on that generation are a non-standard pattern, and replacement, if you can even get them, will cost similar to just getting a new crank.

If you are looking to keep your budget in check, why not look at SLX, almost as good as XT at a significant cost reduction.

The only thing you haven't mentioned, is the frames BB spec, as this will determine if you can run a HT2, or if the BB is BB30 you can't without adapters.

The builder likes to use BB30 so he can make a beefier bb junction but I'll be using a wheels manufacturing adapter. I still might see what my cost is on a new xt crank but I have a soft spot for the m960 crank: