These little guys are great! I have one in a 5 galllon myself, you really have a lot of options for your setup. Basically I'd agree with fella, get some sand or other fine grained substrate. Plants are not really that crucial, what ever you want that will work with the hard water, I always use plastic but only because I haven't been able to grow live ones well. Make sure to monitor the water temp and quality continuously, the puffers don't have gill covers and this helps make them more succeptible to disease, and poor water quality. Feed on brine shrimp, bloodworms, really you can try many different frozen foods my dwarfs have never been too picky. Also it's really fun to watch them eat snails, before I got my current dwarf, I allowed the tank to become somewhat snail infested, then the puffer spent a few days happily buzzing around hunting snails. You won't be able to keep a snail colony in the tank once the puffer is in there because they will eat them all. I find that most lfs's are all too happy to give me all the snails I want for free. Be advised that you should find a place that you feel comfortable with as this may introduce disease into the tank, I've been lucky and never had a problem yet.
Man I could write a book about puffers, they're so very addicting. once you get one, it's hard to care about any other types of fish.
Good Luck
I know it's alot, and it's hard to read but it'll help
SLC