If the brakes got hot and warped... they crapped their linings into the oil. Then you have a plain pure nightmare to clean all of the crap from the system, particularly the hydraulics. If it was operated that way for any period of time (and it probably was)... I doubt the hydraulics will ever be the same.
I'm trying to remember the specs on them. IIRC, a standard single pump system should make 34GPM @ 2750 psi and that might take a couple of remotes to flow that much. Mega's should make 55 gpm at 2750 psi.... and I mean maintain that pressure at full flow.... not just deadhead a guage to 2750 or make rated flow at no pressure.
I would expect the mag plugs to be either drain or drain/level plugs on the planetaries.
At this point in time I'd demand to talk to the previous owner AND drain the oil... because the previous owner probably didn't disclose whatever ails it so dealer may legitimately not know....