Thats the right pulley... if you a looking else where other than GT2, 20teeth and 6mm belt width is the standard just make sure you get 5mm Bore that is the size to fit your stepper motor. There are other bore sizes like 8mm and 6.35mm etc.
Knowing this you could find cheaper ones else where.
I had a Wilson II with a 200x200 heated bed. I did the same as the BOM so I could upgrade it later to a 200x300 bed and just allowed for the shorter heated bed in the software.
There is a bed plate I made up http://www.thingiverse.com/thing:1639553 but there are probably other setups easier than mine.