You will sometimes find them used on a top entering mixer application, but you will seldom find them in a pump application because of the radial space required. Because of the common stationary used there is the danger of losing both seals if you break the stationary face. The convection tank is a unit you can either purchase or manufacture your self. You can probably manufacture one for your purposes at a much lower cost. Do not use the commercial brand that contain a chemical used to plug leaks in radiator hoses.
Finished product is always acceptable.
A fluid compatible with your product is often used. You might consider a cleaner or solvent that will be used to clean the system. If a flush is being used in the system you can always use that as a barrier fluid. Once again, try to avoid using any type of mineral, petroleum or vegetable oil as a barrier fluid. Oil has a low specific heat and poor conductivity that can cause varnishing and coking problems between the seals.