What type of advice are you looking for? how to spend the money? what equipment if any do you currently have for the 125?
I would use the 55g as the sump to save yourself some cash.
Upgrade your skimmer to NWB-200 Reef octopus - 290$ from BRS
Get a Mag return, this will be 700gph return so that will be 50g less then your skimmer allowing your skimmer to do its job. 90$ may want to go bigger to match your head hight. you can calculate this just get something thats going to match the flow of your skimmer.
get some power heads ~3-4 that will cost you like 160$ if you go with korillias or something like that.
90 lbs of rock will be just fine and would allow for a nice open rockscape. I personally wouldnt have more then 90 lbs I had like 60lbs in my 75g and a very heavy bio load and everything is just fine.
If this is a reef tank you could go with the dimmble taoratonic leds and get 3x of these and that will cost you 140$ each so 420$
so that leaves you with about 2 grand to spend on the tank+stand and that should be more then enough so you would have some extra cash for things like carbon/GFO
reactor, sump baffles, plumbing, heaters. I would get a controler personally and I think you would have enough to get one. Get a Reefkeeper Light Plus - 269$ and worth every penny