A lot of the suggestions here about taking classes or learning before jumping into this SHOULD be heeded. I have built kilns to be used with limited resources in situations like you mention, where not much is available, however, there is no short-cut to learning kiln building, firing, and clay work all at once.
The closest thing that I've done to a high-fire home-made gas kiln is the Shopping-Cart-a-gama. This was a collaborative kiln project built using a shopping cart as a frame. I needed a portable kiln that could be fired off with a weed burner and a 20 lb propane tank. The kiln is insulated with soft brick, fiber frax, and steel flashing on the outside to protect from the weather. The stack is 9" steel duct, and a frax-lined mailbox was added to the throat of the kiln once I realized the propane wasn't full combusting before reaching the pots (this helps heat more efficiently). I fire this kiln to c10-12 and add soda ash through the throat of the kiln and through a spy-hole. I have once-fired in this kiln, about half the time, and it does fine as long as the early portion of the firing is gentle. I can get a firing down to ~2 hours from green to c10, however, the firebox is fairly small. The shopping cart interior dimensions minus the fiberfrax and softbrick leaves enough room for about 12 beer steins. I'll generally use about half a propane tank per firing. I also found that I need forced air to compensate for the shorter stack I was using (like an artificial draft), a box fan worked fine for this.
In regards to Sheffield Clay, I love them. I live about 3 hours away, and drive out, getting ~500 lbs at a time (or whatever my vehicle can handle). I highly recommend their schoolhouse white. It's smooth, white, and forgiving. It doesn't perform as well on larger forms, but for functional wares, it's absolute tops, especially for the price. S-14 is a great groggy stoneware, great for hand building or throwing and it stretches great, without complaint. I have had less than a positive experience with their W1A, but that's in pushing a bit above the c6 rating of the clay.
Good luck in any case...