I don't know much about salt water tanks, but I know 30-40 gallons is more than enough if you wanna keep it small and affordable. In fact, many people keep "nano-reefs", which can be as small as 5 gallons.
Many filters are able to be used in both salt and freshwater applications, such as Eheims and Fluvals. Protein skimmers are special saltwater filters that take dissolved organics out of the water before they can decompose.
Saltwater tanks are generally harder to keep, but exceptions can be made with size (I've heard larger ones are easier because fluctuations don't have a widespread effect).
Damsels and Clowns are good fish for smaller saltwater tanks, and they are much less expensive than other fish. For instance, the always popular Powder Blue Tang (Acanthurus leucosternon) can cost well over 50 dollars.
The major drawbacks of saltwater tanks are the high prices and the demand of matinence. Live Rock can easily cost $6 per pound (usually 1 pound per gallon) and live sand can be nearly as much. Also, corals are usually at the cheapest approximately 10 dollars each, which can seriously add up if you want much variety and volume. However, saltwater tanks are WONDERFUL to have and a pleasure to look at and enjoy.
I hope this helped
aaron