Funnily enough swimswim, I'm thinking about doing the same thing with my 180 litre tank. Much as I like my South American puffers, they are rather mindless, and keeping them is a bit like keeping tetras: they're pretty, but they're not pets.
What I suggest you do is beg, borrow, or steal a copy of Aqualog's puffer book. It contains descriptions and photos of practically every species you're likely to see. It isn't an expensive book for what it is (about 18 Euros).
Right now, I'm looking at some of the 15-20 cm Tetraodon species, specifically T. palembangensis and T. suvattii. I have to hold onto a venerable Panaque that I can't bear to give away, so brackish isn't really an option. If it was, you might get away with Takifugu ocellatus, which gets to about 15 cm and while having a poor reputation seems to do quite well if kept cool (not tropical) and in brackish (not marine or freshwater).
Cheers,
Neale