I recommend wiping the handheld since you bought it used. In your case it might be faster to simply upgrade the OS and don't back anything up. Visit
Nextel | BlackBerry Solutions and select Software updates for 7520... link in the lower right. On the page that loads use the drop down to select 7510. You'll want to download the current version of the desktop manager and the OS for your phone. After everything is installed, I would reboot your computer. Then launch the Blackberry desktop manager (BBDM). When prompted for a profile just click cancel for the time being. Now use the USB cable that should have come with your phone to connect your device to the PC. Next double click on application loader and run through the wizard to update your device with the latest applications. Be sure to click on the advanced button so you can reload the radio software and choose the do not backup apps option.
Once this is done, you should disconnect your device from the computer and let it boot. This is going to take some time, usually around 5-10 minutes tops. After the device boots, turn on the wireless antenna and wait for NXTL to show up in the upper right corner. Next you really should perform a master radio reset to force your wirless device to look at the SIM again. After master radio reset I would pull out the battery for about 30 seconds then register the host routing table. If you need help with these items let us know. After all of this is done you should be golden.