My advice is start studying the tank types and equipment, read the ads, read the reviews, and ask questions before you buy anything. You're wise to figure what you want first, and then buy. Second-hand tanks often turn up for a song---BUT---if they're plumbed wrong or if the offered equipment dates from 1980, not so helpful. Go for 'reef ready' or be prepared to take a drill to a tank yourself. COnsider whether you want acrylic because of how heavy a tank that size is---or glass, because it's easier to clean. There are soooooooo many tradeoffs. For the tangs you like, you'll need a 'long' tank, as opposed to wide or deep---they're runners, and swim fast. You sound as if you're prepared to go for it, and have an idea of the costs. We have a red-arrow sticky file up there that will help you. Meanwhile question every item of equipment as to price and value, because, eg, skimmers can cost hugely, and you need to know how much skimmer you need. Lights ditto, especially for a long tank, depending on whether you're going for bare rock or want corals in the picture. The more you know about your real needs and what the tradeoffs are (and the more you know to ask!) the less likely you are to make an expensive wrong purchase. You don't need top-end 'everything' for a fish-only; you don't even need it for most corals: something may be touted as excellent, but the question will be whether this tank actually needs it to work well for the vision you have.
