You get what you pay for with protein skimmers. The top name/price range protein skimmers all perform well and what you pick will be determined by your own setup and preferences. The 200$ range product performance can very greatly from skimmer to skimmer.
So post some info about your tank as I am sure everybody will want to know what you have and to help adequitly answer your question about the water parameters. BTW include your bio load and also how long it has been running if these apply.
As for ammonia, nitrite, nitrates, phosphates, and to an extent ph will all be determined by the amount you feed the tank. Everything except ph should be 0. Nitrates in a reef can be around 10 or so but is really unnecissary as it is easy to keep them welll under 5.
Ph is controlled in part by keeping your alk up. If it falls below the normal levels it can taper off. Not having good airation will also affect the ph. As CO2 builds up it will tend to go more acidic. I use reef buffer to help maintain a stable ph as it is formulated to bring the ph slowly to 8.3.
This is all basic info so if you have a specific question then post away.