When it comes to fish, you don't need much in the way of light, just enough so they can see their food, and you can see the fish. Even so your going to need some form of lighting. There are ways you can work around this. To get things going you can get some inexpensive LED strip lighting, or if you think your going to need 4 LED pendants for the complete tank, you can add them one at a time and spread the cost.
As for tangs you do need a large tank. To a point tank length is often better that tank volume. Figure on a minimum tank size of about 125 gal and consider the weight of the system by the time your all set up. It's going to be well over 1200 lbs by the time your running the system. Yje thing sbout this is that the tank itself is one of the lower cost items in a SW system, so you might as well go with a bigger tank. You will not save that much going a little smaller.
Your going to also need a good filtration system. In short, the main difference between a fish only system and a reef system is the lighting. Your still going to want most of the rest of the equipment. In my opinion, always buy top quality equipment. This doesn't mean most expensive. Obviously, for quality, you'll need to spend some, but you can often find quality equipment that is reasonably priced.
SW reef systems can get expensive fast. If you get everything new, I usually figure that by the time you get all the gear and stock the tank your going to spend about $35 to $50 per gallon of water in the display tank. In other words, a 125 gal tank is going to cost you about $4375 to $6250 by the time your done. You can save some money by looking around for a used system. Often people getting out of the hobby will sell their system at a small fraction of a new one. You can also save a lot if you are into DIY projects. There are also some other things you can do to cut costs. Also, you don't need to get everything all at once. Just get it as you need it. Good luck!