I have owned a TX200 HC and now own the TX200 Mk 3-only changed because I bought a dedicated left handed model although the Mk 3 being slightly longer is easier to cock. Very pleased indeed with both of them-possibly the best springers on the market 'out of the box' if you like a heavier rifle-otherwise the break barrel Weihrauch HW95 would be a better bet.
I know that my original HC had a separate screw- in silencer but thought that was an extra-it certainly would cost more like £30-40 rather than a tenner. However, to me, using it destroys the point of having the HC because it adds about 7" to the length of the rifle. The 'Plenum' I think refers to the built in one and the screw adapter which goes in the end of the barrel enabling the larger 'add on' to be screwed in. Others should tell you if that is wrong.
Hope that is 'sort of clear'-you will certainly not be disappointed with the rifle and probably should wait to see what you get when you take delivery before shelling out more cash.