All models of Titleist balls can be used with any swing speed. Do some reading on golf ball fitting!

Value wise, the NXT Tour is  awesome. You will love the flight and control. Not as thin and soft of a cover than a PROV1x, but still awesome.

Yes, anyone can use the ProV1x, it is not just for fast swingers. It actually flies a little higher this year, and IMO an amazing ball.

Swing speed doesn't matter my friend.  The X or even the Pro V1 can help if you need spin.  Ball fitting is all about finding the right ball from about 50 yards and in based on performance...getting it close! 

Then we can tune the rest of your clubs during a fitting to take advantage. 

 WIll the feel be too firm?

Well it shouldn't feel hard but it is the harder of the two models. you might want to try the pro v1, the spin is a little higher and the compression is a bit lower so that may work better than the prov1x. I initially used the v1x but after trying the v1 i think i prefer that. I get more spin and i believe i get more distance. my swing speed is 95-100mph and my handicap is around 12. Goodluck ball hunting :)

Think about how many shots you take, and then how many you're swinging your hardest for.  Most shots, and the most important shots (relating to scoring) happen around the green 50 yards and in.  Pick the ball that holds the green and reacts the way you want it to on chips and putts.  If you can't afford it, try to find a ball fitting van.  They travel around all over the place and will give you a free two pack of each ball if you want it.  Best of luck with the search and the game.