First, I'd like to list my qualifications to answer your questions.
Front
Older shot when it was my living room
Behind the scene
So...where to begin with your questions. I did the same thing you're thinking back in 2001. I had a 29 gallon tank, and told my wife it would take $200.00 to convert it over to saltwater. I'm a tad over budget as you can tell (but only by 2-zeros).
Let's start from the beginning. What do you want? A salt water fish tank...or a reef tank? A fish only tank is a great way to start...but if you want to start keeping coral (or plan to get to coral) you want to progress in a path that can get you where you want to be.
2nd, what size tank do you currently have, or plan to have. Is it drilled? Can it be drilled. This will tell you what you can have...and what it will cost for setup.
3rd, are you willing to put in time? I'm not saying constant maintenance, but there are husbandry items you cannot skimp out on like water changes, cleaning equipment, etc. They have software programs out there to help keep up with the reminders. They also have equipment out there to handle some of the tedious tasks...as an example, my system automatically tops off fresh water as it evaporates, a UPS will run some pumps in a power outage, and email me if any issues occur.
Once you get through some basics...you can take it as far as you want to go. It all depends on you. For me, I'm glad I made the switch. My tank has had it's ups and downs like anyone...staying with it is the hard part.
Ask any question...I'll answer what I can