Filter? Who uses those any more? Berlin method baby. A good protein skimmer, lots of live rock, a sump for your refigium if you want, but I like a Deep Sand Bed instead.
You can use ocean water, if can go five miles out and get it. The stuff along the coast is generally too polluted.
Salt is not going to run you that much, remember, only initial setup will you need enough salt for all 125 gallons. Make up water is just plain water, and you only do 10% to 25% water changes.
If you'll getting into coral, lighting is another thing you'll need to consider. Rule of thumb is 5+ watts per gallon.[/quote]
I'd agree with everything here except that "5+ watts per gallon" rule... lighting depends on what you're keeping, and what type of lights you're buying. Metal Halides will give you a lot of light in a little space but you could have two 250W bulbs over a 4 foot tank and have the same 500W's worth of VHO lights and you WON'T get the same effect as far as growth.
Lighting is a per basis opinion...