get the tank all set up before the discus get in there. If you add stuff to the tank they will just re-establish new territories anyhow with the new scape. Discus are schooling fish so it isn't really a matter of territories anyhow more a matter of dominance.
I don't think beefheart is needed at all especially if you are getting larger fish (but many will disagree with me)
50 gal barrel?? think home depot 50 gallon garbage can w/ lid.
BEST PART- discus, just like angels, neons, rainbows, plecos and the rest will do perfectly fine in whatever water comes out of the faucet (after being treated for chemicals, temp and microbubbles) ----pH, TDS and all the rest do not matter, your fish will acclimate and be fine forever as long those conditions are kept stable. RO is not needed unless you are trying to breed. peat is not needed ever