Are the puffers in freshwater at the lfs? If so, then you will want to start them off in freshwater. Slowly add salt every week with a water change, until you get to your desired sg. Make sure you use marine salt such as Instant Ocean and not aquarium salt!!!!!
Other than your basic filter & heater, you are going to need to get a hydrometer or refractometer to measure your salinity. Puffers need lots of decorations in their tanks to help break up the lines of sight. If they can see from one side of the tank to the other, they will get bored really quickly. I put a bubble wand in my puffer tank and the puffers liked to 'ride' on the bubbles.
They also need a steady diet of snails to keep their 'beaks' worn down. I'd suggest getting a small tank to breed snails in. Every few days I would put a few snails in the tank, and I would throw in some ghost shrimp every once in a while for crunch value. These foods are very messy though, so you have to be prepared to do water changes and gravel vac about once a week.
Also, IMO mollies are not good tankmates for puffers. The mollies are way too aggressive at feeding times, and my puffers were really docile so they wouldn't get any food. I kept my puffers with bumblebee gobies, dwarf platys and olive nerite snails. Bumblebee gobies are really cute and comical, but every once in awhile one would disappear. The dwarf platies worked out great, and their babies made a nice snack for the puffers. Olive nerite snails are brackish water snails and they have a 'trapdoor' so that the puffers couldn't get to them.
Make sure you do lots of research before you get them, as they are not easy fish to keep. They are very personable though, so they are well worth the extra effort
