Conceptually doppler is as simple as one of these (which is like $3), where the sender and receiver are at opposite ends of a hdpe tube. A faster crystal oscillator can help with resolution/accuracy. We are talking a couple piezos and a trip to home depot for the hardware, but some bench time with graduated cylinders/etc getting it right.
note: with a return line, low flow accuracy might be less of a concern for the turbines if you use two (and they aren't too restrictive). I don't know if pressure differential will work at low flow, or if it does if it will work at high flow.