Is there a compressed gas device designed specifically for creating infusions?
1) I do not think so, the technique was described in e.g. modernist cuisine, they suggest using a ISI siphone and if I can remember correctly does not describe any other tool. Any pressure chamber would work, if you have access to one :-)

2) I have something like this which can be charged with both soda and cream charges, that is what I woudl suggest. (mine is not ISI). Think both you show will work since both charges are the same.  Charge with N2O (cream charge).

3) For the infusion part, you want pressure, look for size of the device that suits your needs. The bigger, the more you can do at the same time, but will require more charges for same pressure.