I've used a Hyper Hammer 120 for a number of years to play competitive squash (Ulster Squash League) and found them to be excellent. The extremely light weight makes them very easy to manoeuvre, especially at the end of long games when my arm gets tired. The hammer design (head heavy) offsets the light weight when hitting hard drives and helps provide stability on drop shots.
I would normally expect a set of strings to last a season (25-40 games) and my current racket has been restrung twice. However, this can vary wildly and depends heavily on how hard you hit the ball, the quality of the strings and other variables like the humidity of the courts.
Specialist racket vendors will sell you a racket unstrung so that you can pick a good quality string and select the tension you desire. Buying a pre-strung racket, the strings will probably be fine but if you are unlucky and they break after 20 games thats the price you pay for a bargin