Just something to keep in mind about do a reef or live rock with fish is the price for live rock for a 125g tank. LR (live rock) typically runs between $6-$15 per pound depending on what type and were you get it. That alone will $750 - $1500 and you havent purchased any fish, inverts, corals or equipment. Not to mention the lighting requirments for corals is going to run you another $300-$500.
Not to be a buzz-kill but the cost of a 125 gallon reef tank could run you in the area of 5k.
You could do a saltwater fish only tank but you'll need to buy alot of fake decorations to make the tank look more natural and where is the fun in that.
I would suggest setting up a freshwater planted tank to start. A freshwater tank will allow you to buy more for your money and if you add live plants you can create a very beautiful set up. Plus freshwater parameters as a little more forgiving than saltwater.
As far as equipment is concerned you'll save a good amount of money ordering things online as opposed to going to petsmart or petco (I understand completely if you like to support your LFS just be prepared to pay a little more).
You'll need a heater(s) that provides you with 3-5 watts per gallon depening on the weather where you live.
You'll need adaquate filtration for the fish you plan of keeping. A community tank will require less filtration than if you are keeping agressive fish. If you browse a site like bigals.com you can compare different filter prices. Having more filtration is way better than having too little.
Your substrate is going to be a personal preferrence but if you plan on keeping live plants i would suggestion getting something like flourite or eco-complete. This substrate has nutrients that root feeding plants will need and will save you headaches in the future.
If you are keeping live plants you'll need to have powerful enough lighting to support them. Different plants will require different lighting but typincally you'l' need 2-3 watts per gallon of water, with a color temp of at least 6500k. Most lighting will give you these specs on the package.
My biggest advice to give you is to fully plan out what you are going to do ahead of time (asking in forums like this are a great way to start this). Knowing the equipment requirements you'll need ahead of time will save you head aches and gobs of money in the long run. Do the research first!
Most importantly be patient while setting everything up. Cycle you tank properly, acclimate fish correctly, and dont rush into this hobby. It always takes me months to get a tank set up and running the way I like it.
I hope this help and to others; feel free to correct me. Im no expert just an expereinced fish keeper
