Ok well first you need to decide what you actually want to keep as far as species, corals.
Generally the canister filter will cause isssues with Nitrates in a reef so I would have 1 on hand for the emergency running of carbon.
For a Reef you absolutely need IMO
1.5-2 lbs of LR
per gallon----can buy dry and seed with live to save. This is what I would do. Buying all LR
is a waste and a hastle for hitchikers IMO
Lighting-----depends on what corals?
Good Skimmer----ask first, some are good, some are not
Good Flow----For a 125 maybe 2 MJ MODS, Seio 1500's, Closed loop etc
Sand or BB
Sump/Refigium? Highly beneficial, I rec. having tank drilled by a pro for such
There is more but these are basics.