Start with your survey. Make a copy and bring it to the county health department. You'll likely need a perc test done to tell you where you can place the septic. If there is county water, the engineer there should be able to get you started on that process as well. If not, you'll need to find the requirements for a well. The health department should have a list of good septic tank people and drillers. If not, ask around and get recommendations from real estate agents and landowners. If you are drilling a well, have the site witched. A dry hole is very discouraging.
Once you know what the septic and water situations are, you have a much better idea where you can reasonable place the home. Contact the electric company for details on what is required for power hookup and a site inspection and cost sheet. You'll likely need to set a 200 amp trailer pole and do all the wiring from the pole to the doublewide according to the electric company specs. If you are away from the main power lines, the cost of getting power to the pole can easily exceed $20 per foot.
Then you'll pull any permits, have the land cleared -including a solid entranceway for the trucks delivering the home, pay for the perc test, get an ok to set the tank and leach field for the septic, and make plans for the exact home placement.
The company selling you the home will suggest someone to do the footings according to the manufacturers specs. Try to have the person installing the septic tank dig the required trenches for these (make sure you measure very carefully where you want the trenches beforehand). Concrete is poured in the trenches, so you want any underground work, like underground electrical conduit, sewer connections, water piping, gas piping, drains and draintiles already in place before the pour. Timing and coordination is key.
Once the footings are poured, the septic in place, and the water and power hookup ready, the home will be delivered. One section will go in place, be put on the primitive pillars, and the other put in next to it and sidled up to the first one using a transfer lift. The company will then join (bolt) the sections together and water level the interior. Depending on the unit there will be two to five days of fussing around, installing interior doors, joining the ceiling, roof, and floor, cleaning and fixing problems. Key issues are making sure the pillars are in the right spots and the load of the unit is properly and evenly supported. Once the first round of fixes is done, you should have some time to inspect and make a punch list of problems. The list is submitted and the second round of fixes are done, usually with you there. All of this goes fastest if there is power and water that can be hooked up. Once the fixes are done, a crew will come in to place the wind straps that hold the home in place during strong winds. Once THAT is done, there may be a state inspection and certification. Then you hire someone to put in underskirting and lay plastic ground cover under the unit. This is a time to also "kill" the land underneath with insecticide, rodenticide, and herbicide. (It can be done earlier, but the layer can get disturbed with the work going on, and it isn't nice for the workers.)
Finally, you'll add the permanent stairs and decks and railings, and have the insurance inspection.
As for what to buy, you get what you pay for, and you want to find a reputable dealer. Expect to pay about $50 per square foot, and remember that you will have all those other costs above to add on. You are fairly close to the coast, so consider contacting MEMA (Mississippi Emergency Management Authority) and inquiring about the units that they had built to withstand hurricane winds.
This company
Deer Valley Homebuilders - Welcome built a series of them and has other plans for homes that are solidly built. Take a look at the photos - these aren't the doublewides of a few years back.
This is their spec sheet for multisection units.
http://www.deervalleyhb.com/img.asp?...n=ID&id=150955
You'll also need to allow money for the heat pump and installation. Homes usually come without those.
Our home has been remarkably problem-free, quiet, and low cost in energy usage. With 2" x 6" exterior walls, hardiboard exterior, and the double roof ridgebeam, this place is stronger than most stickbuilts.