I bought a 10gph unit from QWW about 3 years ago. Good value and good after-sales help.
You will need the following in addition to what comes in the package:
1)Low-pressure polyethylene tubing (red, clear, blue) and fittings.
2)Activated charcoal filter, if not already included.
3)Perko seawater strainer.
4)Check that the included 10ft of hi-pressure hose is long enough.
5)Sodium metabisulphite powder for "pickling" system when not in use.
Sediment filter housings are available with a "burping" valve on the top. These are nice as you can change filter elements without sending lots of air through the rest of the system. Unfortunately QWW does not supply these as standard - might be worth asking though.
If all else fails, read the instructions
If it ain't broke, don't fix it