You want to be a pentester? IMHO, you need to learn as a basic set of skills, the following:Where can i go to learn the basics, like basic networking
Linux and Windows to a system admin level.
Network Devices, their functions and some basic commands of these.
Popular applications and how to configure/set them up (web, dns, mail etc...)
Once you've gathered that information, then start with the computer security portion of your education: How to secure various O/S's, understand access control lists, firewall rules (i.e. iptables), understand various threats against each O/S and network devices, and various popular applications. A basic knowledge of packet analysis is very useful at this stage as is basic scripting languages such as VB, bash, perl, python or ruby.
Then once you have a knowledge of those concepts, you can apply that knowledge to testing these systems and devices as to whether they are secure or not.
I have been told the offensive security courses are very good. I would not take them until I got to this last stage (application of knowledge), that way you would get the most for your time and money.
Luckily, the internet has most of the pre-requisite information for free!
As a start, IBM has a "redbook" site that contains in-depth information about various protocols and configurations. I would recommend the redbook on TCP/IP to start your education on TCP/IP and networking. The link is HERE
For networking devices, Cisco has detailed documentation all over their site on their various products.
You seem to have the basic equipment at your disposal, so I would recommend you start reading and practicing. There are tutorials and howto's all over the net. Start at step 1: "how to configure X" before you jump to Step 3: "pentest X"
Good luck, it's a great journey if you are willing to dedicate the time.