Dizzy, you are def asking very valid questions and looking at the right areas, however, they are way too general and broad to give accurate answers to. the best thing i could recommend is reading through this starter thread.
We have a series of articles to help get you started
youre best served doing as much reading/research as possible then coming up with a rough gamelan of what you want to accomplish. just as an example, are you more interested in a nano setup or a bigger tank? what type of setup do you ultimately want? ( peaceful community-reef or non-reef, predator tank-usually non-reef, etc) as for the cost of the setup, equipment, and necessary supporting equip, that is strictly based on if youre willing to get started with used equipment, brand new, or somewhere between. also if going new, the level of quality will determine price as well. ive personally found that its easier (for me atleast) to buy an entire used setup, then gradually upgrade the equipment as i see necessary).
mixing fish- again this will depend on tank size and other tankmates. most salties will agree that you should never mix multiple "round" tangs (sailfin, desjardini, yellow, purple, etc). but in a large enough tank, people have done it quite easily without issues.
min. equip- another area that needs defined better. a GENERAL rule of thumb is that tanks 55g and under usually dont need a skimmer (unless its a heavily stocked coral tank) and tanks 55g and up do need one. however theres exceptions both ways. powerheads are an almost certainty. also the type of filtration you will rely on changes with tank sizes. i personally havea 125 that uses a 55g refugium below the tank. (i made myself, cost $80 considering the new 55 tank....saved money
so check out that thread, read the links. and see if you can come up with a rough idea of what you want. we would prob be a lot more useful in helping if we knew which way you were headed.