Great score. My first impression is that the rabbits have chewed away all of the sealants. Before even filling it with water let us know if the silicone on the inside corners is all still completely intact. You would hate to find it sprung a leak in 2 or 3 months. Depending upon what type of soap you used I wouldn't worry to much about it. Dish soaps are easy to rinse away and will only add phosphates to the tank. Bad but not bad enough to hold you back. Now if you used brake cleaner or engine solvents you are on your own.
You seem to have a true zest for learning about the hobby and diving right in. For your first tank I'd suggest starting slowly. I'd suggets easing into a reef by starting out with a peaceful FOWLR tank. (FOWLR, fish only with live rock) Get yourself a nice DSB, deep sand bed, of Carib Sea Aragonite (if available). Buy as much dry as it takes to fill the tank about 4". You can buy a 20lb bag of "wet" to seed the tank with beneficial bacteria. You can also ask around and get a couple of cups of sand from a fellow reefer. Fill the tank with salt water that you have premixed in a brand new trash can that will only be for this purpose. I'd like to say stay away from the wall about 6" if possible. As time progresses I assume you'll be adding an overflow box to accomodate more hardware. After the sand and water have settle out you can begin buying live rock. The stuff isn't cheap so look in craigslist for people breaking tanks down in your area. Do a yahoo search, salt forum, TN Your City Name, and see if any forums come up in your area dedicated to salt tanks. I have 2 sites dedicated to reefs here in Austin. You can usually find people breaking down tanks offering rock for $2.50 a lb. My friend Monica sells it very reasonable at www.oceanhomes.com
. If you can afford 40 lbs at start up you'll have a great start. Add a few devices to add currents, for a 125g I'd recommend Seio M800's, maybe a pair or even 3. For your Fowlr I'd maybe think of getting a nice canister filter, I'd recommend an Eheim 2217 from www.thehobbypalace.com
usually about $100 and that's a steal. You'll think about scrapping the canister as your reef becomes more complete. Now wait about a month and let the tank mature. Have an LFS test your water, Ammonia being hte most important part of the "cycle". When ammonia levels disappear you have a nice tank begun. Add a couple of nice fish that you like, do understand that you will probably trade them in later as your reef fills in. A wrasse, a couple of clowns, maybe a naso tang. Stay peaceful and small if possible. While learnign the ropes you want your tank to stay as clean as possible, don't pollute it with aggressive fish like lions, eels, triggers, or groupers. If you decide that fish are for you and that you don't want the headache and expense of corals you can later make it an aggressive FOWLR with some really dramatic fish. Stay away from the damsel dither story. Damsels are the terror of the reef community and are better used as food for your lion fish than an inhabitant of your tank. As your tank becomes more reef like and cash becomes available begin thinking about adding an overflow box of about 1,600 GPH capacity, a nice sump ( a 55g long tank would be perfect as you could easily silicone in a divider for a 10-20g refugium built into the sump without having to add another pump), a nice skimmer like euroreef or ASM, and a good return pump such as Eheim 1260 or an external pump like Dart to return the water to the tank. Once your sump and skimmer come online unhook the cannister filter. After securing a good filtration system lights become the single most important factor after clean water for a reef tank. A standard 125g long should have 3x 250W metal halide bulbs and about 300w of supplemetal PC or T5 actinic lighting. 3 250w bulbs are better than one 400w bulb as each light only spreads out about 20". Once your lighting and filter are running it's time to add more rock. Get it up to about 200lbs if possible. You could save money at this point by buying dry "live rock" as it will seed from your existing tank. After your lights, filter, rockwork come online your DSB should be fully matured and it might be time to start thinking about corals.....
Of course with a Fowlr much of that is not necssary. A good canister or maybe even 4 of them. I have 4 on my 125g. Any old flourescent strip lighting will do as your fish don't even really need the light. a good water circulation via power heads or stream makers like those I listed earlier.
To test the tank I'd definitely set it up somewhere and fill it to the top. It must be a perfectly flat and level surface that can hold 2,000 pounds (your tank will weigh approx 1,080bs full of water) or the tank will warp and crack. You'll need to bail it with buckets or use a powerhead and some vinyl tubing to get the water out. What to do with it? water the garden. You don't need to test it with mixed sea water, plain old hose water will work.