The cog is basically a good kit, with the main problem being that the deck planking isn't laid athwartships, which according to current thinking it should do; also, there are cargo hatches which shouldn't be there, as the deck planking supposedly was removable. One has to take into account, however, that we don't know as much about these ships as would be necessary for a perfect reconstruction, so perhaps there were indeed cogs with deck planking like in the Zvezda kit.
By the way, the kit seems to be based mainly on iconographic evidence from the Baltic rather than on measurements taken from the Bremen ship.
Just the other day I talked to someone who is in the process of putting one together, and there seems to be one area demanding attention, and that is the fit of the lower hull (if you don't want to show her waterline, that is); he apparently had some problems keeping the planking detail in order.
The crusader ship is a Hellerish marketing ploy - it is the cog with some modified details and the addition of a small boat that is also separately available. Although the term "crusader ship" is sufficiently vague to allow interpretation as a ship of the Teutonic Order in the Baltic, the intention was probably to put it into context with Zvezda's figure offerings coverng the crusades. For that purpose, a cog isn't the proper ship, though.
As for where to get them, you might try that auction site where they sometimes surface at considerable less than list price which I find to be a tad hefty.
Jorit