Sand bed filters are excellent pure biological filters, because the sand is fluidised by the water flow through it then every particle of sand is able to get at oxygenated water going through the bed this means a massive surface area for the good bacteria to colonise.
The constant motion and knocking together of the sand particles means that a film of bacteria is always present and will always be working because there is no crap left to obscure the media.
You can buy sand specifically for the fluid bed, I am not sure how if differs from normal sand.
The water has to be prefiltered first to stop grot getting into it.
You can set it up exactly as you said as a free standing system but the pump has to be capable of creating enough head to
1. get the sand fluidised
2. return the water back from the filter.
3. You dont want the sand exiting out of the return either so as you said, it needs a flow adjuster on the outlet.
The bacteria will die off at the same rate as the bacteria die off in a canister filter as they use the same nutrients, oxygen upload etc.
I have often thought of using a sand filter on my tanks purely for bio fitration, just have been too lazy to do anything about it.
Good luck Richard, it will be a good addition to your filtration but as you have HUGE filtration already it is not a neccessity, just a nice to have that will certainly do no harm.
Steve