Some thoughts:
Figure on 3,000 pounds per yard, so five yards is 15,000 lbs. That's 250 60 lb bags. My Home Depot has them on sale for $2.17/bag if you buy a pallet or more, which is cheaper than the price I could get from a truck. It's also cheaper per pound than the 80 lb bags. I prefer the 60 pounders because they're easier to handle.
Foundations are usually done in what's called a "monolithic pour." What that means is that at the end of the job you have a single piece of concrete. Cured concrete doesn't bond well to uncured concrete, so time is of the essence when doing a monolithic pour, once you start you can't stop. If the concrete already in the form has hardened before you add more concrete you get what's called a "cold joint" which can be a weak point. So you need to either plan it so you can work uninterrupted, or figure a way to break it into separate job. I would say that 250 bags is a lot to do all at once, especially if you aren't experienced. What happens if your mixer breaks down after 100 bags?
Look into piers. It's a big advantage to be able to do them one at a time. If you have stamped plans your engineer should be able to tell you if they're feasible with your soil, how many you'll need and how big.
The other thing to think about is how you're going to get everthing else that goes into the cabin there! If your cabin is 1250 square feet, a rough guess would be 20 pounds per square foot for the materials to get it closed in, or 25,000 lbs. That's a lot to take in small loads.
Keep us posted, these kinds of projects are fun.
