solar panel to d.c motor power conversion
So for that what will be the situable rating of the d.c motor to
  achieve a speed of 3000-3500rpm


Really tiny motors (a few watts) can certainly produce many tens of thousands of RPM. Speed is one half of the quantity that "makes" power. Output torque is the other so: -

Mechancial power out = \$2\pi n T\$ where n is revs per second and T is torque.

If your solar panel produces 50 watts then you cannot exceed this figure unless you operate your 250 watt motor at a duty cycle of 20% but, this requires an extensive controller and usually a battery that the solar panel keeps charged up.

As for voltage, that depends on the panel - if it doesn't produce enough voltage then your motor will turn more slowly.