Regards the Offsec course. There was a thread just recently discussing it, and a few people who have attended the course made some pretty detailed comments. In particular, I went over the circumstances under which I do and don't recommend people take the course. Your first test (should you choose to accept it) is to find that thread. You do need to be able to track things down on your own to be successful in the PWB course.
Next - do you have a good understanding of TCP/IP? By that I mean have you spent time looking at packet captures, configuring IP clients and servers, troubleshooting communication problems, etc? Have you attended any TCP/IP focused courses (e.g. CCNA) or read and understood any good TCP/IP references (The Richard W Stevens book for example). Know what DNS is and how it works? Have you run a port scanner or a vulnerability scanner before? Do you understand how they work? If so you should probably be OK for the information gathering/scanning phases of the course.
Third, try the free Metasploit Unleashed course. If you can follow all of it (by that I mean understand whats going on, and be able to take what you learned in the cours and apply it in different situations), then you should be OK for the exploitation portion of the course. Next Id recommend trying some of these buffer overflow exploit tutorials. If you can follow at least the first one or two you should be OK for the exploit writing sections of the course.
If you're OK with all of that you should be OK with the Offsec course. It probably has higher entry requirements than most other similar IT security style courses, but if you're willing to put in the effort you should be OK.