You'll definitely need a fitting with that much speed. 125 mph is faster than 95% of the guys on tour.

Go find a good fitter. That is the only way to get the right equipment.

The one that gives you the best numbers.
Seriously, if you're going to spend the money required for a new driver, you need to get a fitting at best, or try out several options to find one that fits your game.

We can’t give you a good answer.
The -2 is the most forgiving, the-3 controls spin better, and the -4 reduces spin the most.
At a swing speed that far over 100, the off the rack driver shafts probably won’t be optimal.

If you can find a Titleist sponsored fitting or a Premier fitter under fittings on this website, they can recommend a head based on your swing and offer shafts suitable for your swing from the library of no upcharge shafts as well as the ones that include an upcharge.

Depending on whether you hop on the GT model now or wait for a GTS, Titleist drivers can run from $450 to $850.

Like mentioned above, you're crazy if you're not getting fit at those speeds. That said, I'd be looking at the GT line with the price drop. That's my SS range and I'm gonna tell you, the stock shafts probably won't do it for you. The fittings I've gone to have all been the same. The fitter goes straight for the Tensei 1K Black and I stop him before he even gets to the rack. I hate that shaft so much, so the alternative is shelling out another $400 for something that suits you, so the break in price on the GT series will soften the blow a bit.

125MPH!!!! Wow
I'm just around 105-108 and I can send it over 300 with a GT2 I can only imagine how far the ball travels when you hit it at 125MPH