Adding on...
1. Maintaining will incur a lot more expense as listed above. Also there is a much larger start up cost.
2. As said before, its not really that much more maintenance, but you need to be diligent.
3. Here's a basic list of some of the equipment you might find on a saltwater tank:
Sump
Weir/Overflow Box
Protein Skimmer
Powerheads
Lights (VHO,T5,MH)
Live Rock/Base Rock
Sand (a lot of different types as well as SSB,DSB,BB)
Saltwater (ASW,NSW)
Foods - Some common ones are mysis, enriched brine and spirulina
RO, DI, RO/DI filters
Quarentine Tanks
Refugiums
I've no doubt missed some out but I think thats a pretty comprehensive list anyway.
4. Yes, however if you do everything correctly, research your purchases and quarentine your stock then problems should be minimal.
5. How long is a piece of string? Things like type of set up, type of filtration, amount of filtration and method of cycling all come into play with this.
6. Yes there are, however I find most of them to be underlighted, underskimmed and underflowed (if those are words?). Your best bet would be to buy a standard tank and buy your equpiment seperately, that way you can customise your tank to your different wants and needs.
So I know I havent answered your questions specifically, but it would help if you did a bit more research so you had some idea as to what set up you'd like to achieve. A start would be researching fish only (FO), fish only with live rock (FOWLR) and reef tanks. These different set ups will then branch into different areas and the requirements will be different for each, which should enable you to ask more specific questions.
If your unfamiliar with any of the abbreviations I've used I;m sure you'd be able to find them on google, which will help you with your research too
.