[DonRotolo]

To receive the signal, use an omnidirectional "ground plane" antenna. Very inexpensive and small.

The DDF signal varies too much to decode data from it, generally speaking. There are ways around it, but not easy ways.

Yes, you may in fact receive signals from more than one transmitter, but your flying algorithm will not simply pick up 'one' signal and fly at it, instead you'll fly past it and note where it was, then turn and fly at it. Or, said another way, you need to find the right one before you can steer towards it.

Try this on a regular map. At one spot, you get the target at a bearing of 110 degrees. At another spot, you get the target at a bearing of 150 degrees. You turn 90 degrees and get the target at a bearing of 120 degrees (all measured relative to the aircraft) again. Draw lines to infinity from those locations, where they cross is the target. Now go there.

Again, this isn't a trivial problem to solve, but I still think it is within your budget and reach. Before committing to air, I'd first learn how to find the transmitter on the ground. By breaking this problem into manageable pieces, what you learn on foot will be easier to use int the air.