Agree with Dave. You want the drain line to go as far as possible away from your well. Just run allot of pipe to get it there!!!
You said that the water is flowing from a spring. Is it safe to assume that the height of the water is at 3ft above the floor of where the toilet will be?
I would just buy a $99 toilet kit from the box store to use. The trick is to make sure the water full line in the tank is lower then the height of your spring. Water will always seek it's own level, so if the tank is below the spring, you can fill the tank with the spring.
Just run a pipe to the bottom of the tank and let the water fill it up. It will be slower then a toilet in a house, but the water will flow uphill from it's own pressure to seek that level from where it came from. When the tank is full, the float will close the valve. It's pretty simple.
The biggest problem will the the height of that tank. If it's close, I would cut out the concrete and mount the toilet flush. You will need electricity for this, but with a generator for power, it's pretty simple to do.
I use a regular 7 1/4 inch circular saw with a diamond masonary blade in it to cut the square that I want to remove. A 3 inch pipe is all you need for a toilet drain, so make the hoe 4 inches . After you cut the cemnet, break it out. I've used cement drill bits to break up the cement and beat it with a hammer and chisel when I've had no other choice. An air hammer with a pointed chisel tip on it works too, but it's slow and noise. My SDS Max rotary hammer works the best. It's a super duper concrete drill that also has a jackhammer mode that just melts through concrete.
Once you have the hole, dig a ditch next to the foundation where you want the pipe to come out and start tunneling. In most cases, I use a water hose, but since you don't have any water pressure, you will have to do it all by hand. It's a slow process to tunnel under a slab, but you shouldn't have to go very far. Keep that in mind when you position the toilet.
I know this sounds like a fair amount of work, but it should only take a few hours if you stick with it. Most people get intimidated about doing this before even starting and end up creating a box to mount the toilet on, then hate it. I've seen this on several homes where they didn't now how to go through the concrete. I'm done with it so quickly that it's almost funny to hear the reasons why they couldn't do it.
Do it right and be proud of it, or do it wrong and regret it forever.
Eddie