1) What kind of filter do I need? (what size also)
This is kind of up to you. An oversized HOB (hang-on-back) power filter should be enough.
2) I have read in some places to use aquatic salt, and others say to use marine salt..which is it??
You want marine mix to set up brackish conditions. Something like this: Aquarium Salt Mix: Salt for Saltwater Fish & Reef Aquariums
3)What kind of gravel is recommended? Sand?
That's kind of up to you, but I think sand would be a good idea. You can use regular play sand from Home Depot. Just be sure to rinse it really well before adding it to the tank.
4) I am planning to breed snails for my GSP. Can they be put in the same tank with him?
Doubtfully. There aren't many options for snails that will live in brackish conditions and you wouldn't be able to breed them in the puffer's tank anyway. I'd set up a second tank for breeding snails. This doesn't have to be anything fancy; a 5g container with a sponge filter should suffice.
5) How long before I transfer my puffer to the new tank, do i have to cycle it?
You shouldn't try to cycle with a puffer, if that's what you're asking. If you want to upgrade tanks, I would get the new tank, fill it with water, move the filter from the old tank to the new tank and then acclimate the fish. Since you'll have a new, bigger filter after a while (a month or so should be enough) you can remove the old filter. Or, you can just leave it on there for extra filtration and insurance should one of the filters die.
6) When do I put the salt in? Before or after I cycle it?
The puffer is currently being kept in freshwater, yes? Assuming you're getting the new tank as soon as possible, I would just set that up as freshwater as well, and slowly convert it to saltwater (going from fresh to mildly brackish and so on until it's very brackish or even full marine salinity). Of course, you'll want to do this over the course of months as the puffer grows, so it would probably be helpful to chart out the increases in salinity over the course of about a year or so. Gently switching will be best for the fish but is also necessary to keep enough bacteria in your tank to handle the fish's bioload.
7)Do they need a heater? Or can I go without one? If they need one, what temp should i keep it at?
You should have a heater. 77-78*F should be good.
8.) What is a suitable salinity level to keep the tank at? SG Level?
Eventually, you'll want a heavily brackish or even full marine tank. I think a salinity of 1.020-1.024 would be a good target (like I said, you'd want to slowly raise it to this over the course of a year or so).
9) How often do I need to feed my puffer?
Daily. Puffers forage for food constantly so feeding them every single day is a priority. Try to feed a varied diet to keep the fish healthy.
10) About how much salt is required for a 29g tank?
Depends on the mix and the salinity. Because you'll be doing water changes, marine mix is something you'll need to buy on a regular basis. I'd just get a big bag/bucket to start with and replace it as needed.