Would it be more economical to purchase a protein skimmer and live rock from a second hand source? (ie. craigs list)? or would using already-used equipment be bad? I just looked up the prices and the ones i saw averaged 200$. yikes!
Live rock is essential - is a protein skimmer absolutely
required? No, but it does help. I have yet to run a skimmer on any of my Saltwater Setups (5 currently, had to cut back). With that said, as long as the skimmer seems to be in good condition you should be fine with a second hand source.
As long as you have live rock in the aquarium that hasn't sat out of a water source for very long (in order to still contain the beneficial bacteria you need) you should be fine to get it from where ever seems cheapest. The more live rock, the easier the maintenance in the long run. Live rock contains all that good bacteria in order to fully switch Ammonia to Nitrite and finally Nitrate; just like an aquarium filter. This is why it's recommended to get as much as possible to help maintain the fish in the tank, and future additions with enough bacteria to support the levels of ammonia being released into the aquarium by the fish. "Dead Rock" or Live rock that has been sitting out for a long period of time will take longer to grow this bacteria back (Once again, just like a new filter) and the cycling process will take longer.
Example: My 20-Gallon took forever to cycle even after adding bottled bacteria (1 month+) to the tank with 15lbs of Dead Rock, however after adding one 8lb piece of live rock (freshly moved from another system that has been running for quite some time) the tank finished cycling within hours of it being added. 3.0ppm Nitrite to 0ppm in just a matter of hours with a little help from some fresh bacteria.