You all ready have access to the staple of most goblin armies. Lots of goblins
You can find some of the heroes very easily on ebay. The biggest thing I'd suggest for you to get hold of would be a goblin shaman. They really are great at holding a goblin army together. The named goblin kings are very good as well
As far as competitiveness goes as well as which units to look at...Get some games in.
Practice makes perfect and getting a good grounding in how to use basic goblins en mass will help later when you have some more elite units to play with.
Some elite options are Gundabad blackshields and warg marauders as these are harder to kill and more likely to inflict some damage.
In regard to a good army, well you can do an all hero force to begin with if you wanted. They are often quite hard to play as they're always outnumbered and every might point has to be spent in the right way but they're also quite fun as well
Happy gaming