Hi,
I entered SW a lot like you, after a few years in FW tanks. I converted a 90g tank to SW about 20 months ago. I'm a HUGE fan of the 90g tank. It is an extremely flexible size that will allow you to house most reef-safe fish as well and lots of room for LR and coral.
I'm working towards a large SW setup, but some things are concerning. To quote my wife, "A small percentage of a big number is still a big number." In a 180 or 210 setup, anything you do will have to be done "big" and the cost really adds up. Expenses that you just don't see in FW.
A 20% water change on a 210 will cost $15 in salt. You will also need a container to mix that much SW in, a powerhead/pump to mix it, and a way to get it into the tank. A minimum of 25x flow in a 210 is 5250 gph. That's a lot of powerheads/pumps. Reefs require expensive lighting and, oh yeah, all this shows up on the power bill.
I'm not trying to talk you out of it. You just want to move into this with the eyes wide open.
Prepare the wife to be patient. I new SW setup can look ghetto (inside the tank) for months. It takes 8-12 months for a tank to start to settle in. My 90g is in my entryway and we had a lot of raised eyebrows and even comments on how poor it looked while the system was getting established. I loved seeing it, but cyano and bubble algae are an acquired taste.
I guess I'm biased, but I think doing one from the beginning is a worthwhile endeavor. I would NOT recommend buying new, though.