Strings that are easy on the arm AND durable? That will be tough to find; probably Head RIP Control 16, or Intellitour 16. Soft and durable, with a dead feel and low power.
Honestly, the Pure Drive is not going to be easy on the arm, and any string choice is not going to change that fact too much. Plus a soft string in that racquet is going to break fast.
The best thing for the arm are racquets that are over 11.5 oz and with a flex around 63 or less.
Head Pro Tour 280/630 MP, 18x20. Tour Bite 1.15/Mosquito Bite 1.16.
Sampras serve, Agassi backhand and Federer forehand. heh heh