If you are going FOWLR then it is possible to go barebottom, which is sort of in vogue amongst some of the larger marine pred keepers at the mo, though as has been mentioned, you lose a very large buffer. However, most fish are more forgiving than corals in water conditions, so in FOWLR it isn't as bad as in a reef.
You don't HAVE to have LR, but it does make it very hard work and costly to maintain, unless you can pipe in natural sea water constantly as a permanent water change.
If you don't like the cost of LR, you can always make your own, plenty of sites (such as GARF's) show you how.
If it is FOWLR then the heavier stocking would make me look for a protein skimmer to aid the filtration.
A decent amount of LR and a skimmer should be all you need, especially if you have a good flow (more than turnover you need to make sure there are no dead spots in the tank).
RO water is really necessary, though most LFS will sell you ready mixed saltwater so all you have to do is pour it in (but you will need pure RO for top ups).
If it were me doing a 30 gallon FOWLR I would be looking straight at a fuzzy dwarf lion, or a fumanchu dwarf lionfish. Then stock around that.