i have a 135 and am already eyeing a 180... it looks like the same size tank but instead of 18 inches wide, the 180 is 24... i think it will give a much better look especially since i can arange the rocks much easier.
And as my sump/w/d/skimmer/and fuge setup, i will be using the 135..
.... and then probably want to go larger after the new setup
...... but that is the key, everyone wishes they want to go larger.... by best suggestion is go with the 55... its large enough to get you started , will require less money for LR/Sand/etc......
make your mistakes with a smaller tank so it wont cost you both arms and legs... just one set is enough..
To answer some of your questions.
W/D - basically a sump with bioballs.
Sump - a tank where you put your heater, water return pump and is usually the place most put their dosing into.
Fuge, basically a tank where you put on lights and grow algae. this keeps more control over your water paramaters as well as other benefits.
Most people integrate these to a certain point. Some get rid of their bioballs becuase they reply on their Live Rock in their main tank to keep the bacteria necessary.
In my current setup, i have a W/D with bioballs (only due to the fact it reduces the noice factor of the incoming water from the main tank... my heater and return pump are in my W/D.. next to the wetdry, i have a 30 gallon tank which is my fuge. from my return pump, i have a t connector with a ball valve which returns some of the water to the main tank, and some to the fuge... there is an external overflow on the fuge which returns the excess water back to the wetdry sump area.... and around the water goes...