1) Yes, different bacteria would need to form, because freshwater bacteria differs from saltwater bacteria.
2) Yes that filter is usable in SW
aquariums. Live rock is usually preferred to have in SW
setups as this provides the aquarium with adequate hiding spots for the fish, looks good, and provides more biological filtration.
3) I would suggest a FOWLR
(Fish only with live rock) set up for you.
4) Decor I would leave probably just at live rock and you can get some neat looking pieces and colors to them. You could also try some bleached coral and live rock both if you don't like the live rock alone. The substrate I prefer is a mix of very fine sand and grainy sand, though you can go with crushed coral, whichever you prefer.
5) Sand (30 - 40 pounds) will run you about $30-$50 depending on the kind you get.
Live Rock I would put about 20-25 pounds of it and that would be probably about $115 depending on where you get it (LFS
Salt Mix - $20 to get started with
Hydrometer - $10
I'll add more if I think of anything else you may need.