I would wait on the O2 sensors since the CEL will let you know when one goes bad and it will tell you which one of the 4 is bad.
As for power you have the 2 main ones inmind already. If you are bolting on a cone filter make sure you have a heat sheild or you will just be sucking up hot air (see the FAQ for some temp readings on this).
http://isuzufaq.ibctech.ca/#faq49
the only other large HP gain is to purchase a supercharger kit (which they do make for your truck).
The weak link in your system is the tranny since it is only rated at like 300hp max.