The WAAS enable GPS I linked to was only one of several that are sold by SparkFun or by others. It was not a suggestion but rahter an example. You will need to look at all the features to decide what is right for you. I think it is safe to say that all the SparkFun GPS modules could be used with an Arduino.
A WAAS enabled GPS does not need anything else to make it work. You only need the single module for your mobile platform.
There are any number of documents, examples, and projects that use GPS that can be used to learn from. Just make sure the example matches your receiver. GSp modules have numerous data formats they send data and take commands in. Anything from the NMEA, TPSI, other text and binary formats, and GPS-ICD-153 for military GPS modules. For your case, I would stick with NMEA and use the Arduiino GPS module to decode these text messages.
I think I would take exception to DPGS not being as good as WAAS. If you are very close to the DGPS base station, it has to be better than WAAS. But on the average across the country, WAAS is likely better.
Forgive me, but your use of English seems just off. If you are outside North America, you need to look at the coverage maps (like here http://www.nstb.tc.faa.gov/RT_VerticalP ... nLevel.htm
) to see if WASS covers your area. If not, DGPS will be needed and then you need to decide what DGPS systems are installed in your area and what receivers are available to pick up the RF broadcast and send the correction message to your DGPS enabled receiver.