Preface: These tips and suggestion are in no particular order, and some will need some fact checking, as they came from various sources that I am not sure of their reliability, but I will make sure to say something like, "I hope this is correct" or "I'm not certain if this is correct" I'm not bringing them up to be confusing, but in hopes that someone
will know if they are true or not, as I would like to know as well.
I am not able to advise you on sumps or skimmers, (among other things) as I know very little about either one, and until I have a carpet-less floor with a drain I'm just personally too afraid to set up a sump. You will of course need a skimmer, I'm just not skimmer savvy enough to suggest the best one for your needs.
As you guessed, marine is going to be a bit different from low end brackish, the hugest difference is price. You can very easily exceed your 2K with a marine set up of that size, but I'll see what I can do to help suggest keeping the price down so you can get this set up and not skimp on the essentials.
Liverock/dryrock:
J-P already suggest the wonderful site Marco rocks, I believe PP linked them in another thread too , I'm actually ordering from them in the next couple of days, so it was something I was going to talk about too.
http://www.marcorocks.com/
Ok, your tank is going to need 1.5 to 2 lbs per gallon. This is what is required to maintain the biological filtration a tank this size. Liverock, (cured quality) as J-P mentioned is expensive. You could easily spend all 2k on the liverock. But dry rock is not so expensive, (about 2.38 a lb ?) but the liverock will eventually seed the dry rock. Yes, you could go with very little liverock, and quite a bit of dry rock, and wait several months to let it seed, this would cut costs down greatly if you want to wait that long.
But that probably sounds a bit discouraging, so if you go with a little more live rock like a 50 - 50 split, more or less, it is possible to cut that waiting time down, and keep the cost down.* "I hope this is correct."
I also heard that when adding dry rock to a tank initially, to keep the lights off/low for a few days to allow a biofilm to grow on the rocks, and this helps deter algae from growing. "I have NO CLUE how much truth is in this statement, but I hope it is true, but it sounds too good to be true. lol"
Sand:
There are debates about using pool filter sand, which I believe is either part or all silica based, aragonite sand, or crushed coral. People are concerned about the unknown elements of pool filter/play sand, and what it will release into the water column, besides silica, but it has also been suggested that silica sand can be sharp and be harmful to certain sand sifters. "I don't know if this is true" People will argue that aragonite or crushed coral should be used because it will help buffer the pH to 8.3, however, with liverock being mostly calcium based, the marine salt solutions being "buffered" to 8.3 pH, so on, it was brought up that their really isn't any "buffering" action going on as the pH in the tank is already high at around 8.3 pH, and that the pH would have to be in the low 7's for this buffering action from the substrate to make a difference. "I don't know if this is true"
So besides all these debatable statements I personally ran across while researching, that I as a hobbyist myself am not in a position to test, I personally just prefer aragonite sand anyway. It's pretty, it's white,it's not sharp; it's not cheap, but I don't like the unknown composition variable of the play/pool filter sands, and things generally aren't recommended by pros if it's not a good product.
Something mentioned by RTR recently about sand depth that I'm glad he did, (Hopefully my paraphrasing doesn't butcher or misrepresent) he says that especially with sugar fine sand, at first it is best to start with a shallow layer about 1/2", and give it time to populate with your beneficial bugs and whatnot and allow their "sifting", and then you can later slowly add more depth. This prevents it from compacting and trapping in harmful gasses, I'm not certain if this is the case with all size the substrates, but it sounds like a good practice to follow regardless. Better to be safe than sorry.
Preventing a cracked bottom:
I think this was a J-P suggestion, place plastic egg crate (available at hardware stores in the lighting department) on the bottom of the tank before adding the sand. This creates a platform for your live rock, prevents rock collapses, and with all the weight of your liverock, the pressure weight of that on the grains of sand alone would crack the bottom of the tank. You can cover either the entire bottom, or simply under where you want to place your liverock.
Buy used when possible:
The tank itself, stand, and lighting, again can easily knock out at least half of your budget, I suggest looking for a used tank. I know that takes some of the magic away of getting something "new", but you need a lot of items to start up before you can safely house your new puffer friend, and you want to make sure you don't skimp on anything. Check clearance online, craigslist, ebay, anything you can think of. Be positive and diligent, and you surely will find a good deal on this. This really goes for any equipment.
Current:
You will need 4, not 1, circulation pumps as PP said for that size of tank for flow. I actually have one of the Koralia 750's she suggested, that I have because she suggested it
Water; mixing, storage, and transport:
You will need a mixing vessel to have your saltwater mixed and ready at anytime. Many people use new, clean, rubbermaid trashcans on wheels. "I hope these are safe because I just bought one upon suggestion" I recommend you get the 50 gallon one because of the size of your tank. You will need a powerhead, heater, and tubing, same reason with the brackish water mixing and water changes. It is highly recommended that you do not used tap water, I hope that there is a way you can transport RO/DI water to your home, or even better, have one installed in your home.
Prevent boredom:
I suggest arranging your rockwork with the puffer in mind. They love to explore and swim into, through, and around things. I am always saddened when I see someone's rock piled up in a fashion that *may* be pleasant to their eyes, but super boring for the poor puffer. Be ready to change the rockwork around periodically because they get bored.
Feeding:
Don't feed your puffer with your bare fingers/hands. Use a designated set of tongs, he will learn that tongs bring him food, and he will likely not bite you when you do maintenance.
Don't break your house:
If you haven't already, double check that your floor can support the weight, you may need to support the floor beneath before setting up to avoid sagging/cracking.
Dry rock to live rock ratio,
Ok, this is a suggestion I am again unsure of how true, possible this would be. So I'll phrase it as a question to the experts:
If this tank is set up with live and dry rock, if he gets a juvenile puffer <5" or so, and no other tankmates first off, how much cured liverock (properly quarantined and everything else required) would/could/should he put in to safely add initially to handle the bioload while it seeds the remaining rock that is dry? If he can do this, is there still some amount of time he should wait, and if so, how long, and what to do in the meantime?
HTH at least a little, I'm sorry I'm not more knowledgeable in this area, and I wish I were "absolutely sure" on certain matters, but I look forward to hearing everyone's input and answers!
but it will cut you expenditures by 1/4.