It definitely does take quite a bit more attention than a freshwater tank (especially in smaller tanks where evaporation can plague havoc on the salinity levels if left to go unchecked).
If you're serious about the hobby, then definitely invest in an RO system, but it really depends on your budget and so on. Not only is the RO great for topping off the tank, but it can be used for other purposes also (drinking, cooking, etc.). Honestly though, for the 8+ years that I was into reefkeeping, I have yet had to use anything but water from the tap (not directly straight out of tap, but rather tap water that's aerated in separate buckets). I do plan to install such a system when I get my own place (being built as we speak), however.
As for placing mushrooms/zoanthids into the tank right after it has been set up, it's something that I wouldn't do. I would wait at least a couple of days until everything has cleared up a bit (aragonite can be dusty for the initial 24-hours that you place it into the tank, especially if you use sugar grade oolite). It's also a great time to make sure that there are no leaks, and everything is running efficiently. It is also dependent on the source where you're getting your initial stock from. There's a big difference from getting them as frags from other aquarist vs. transhipped specimens.
Before you do this however, make sure you read up on things. There's quite a few great books out there (articles/books written by Sprung & Delbeek, Fenner, Paletta, etc.), and definitely check out the reef forums since the hobby really evolves very quickly (make sure that they're reputable, e.g. reefs.org and nano-reefs.com are a couple to start out with).