Hi There,
I take it that you want to measure distance to target not speed?
Do you have available a doppler radar head with a Varactor tuning diode (Type used in alarm sensors,etc)? This must be able to smoothly change the frequency of the unit over at least 300MHz range by varying the voltage...
The company Mini Circuits does a whole range of extremely good VCO's that would suit your purpose, prices are around 50 pounds or so. I use them myself for Radar circuits.