You don't just need a DC/DC converter, you need an MPPT. Look up the I/V curve for a solar panel. It's non-linear...at either no load or short circuit they deliver 0 power. Maximum power occurs somewhere in the middle. Where, exactly, depends entirely on solar irradiance, temperature, etc. If you just stick a DC/DC converter on a panel's output, as soon as the motor draws more power than the panel can provide (cloud passes by, etc), the DC/DC converter will drop its input impedance to compensate for the voltage drop, and you'll fall off of the edge of the panel's I/V curve and get essentially zero power, indefinitely.
An MPPT always runs the panel at its maximum power point (the MPP in MPPT, the T stands for Tracking). Whether or not you have a battery, you must have an MPPT, and at the power levels you're talking about it's not very likely you'll be able to hit your budget.
BTW - most MPPTs include DC/DC conversion. Most are designed to charge a battery, but you might be able to directly drive the motor instead. You should talk to the MPPT manufacturer about that, some are designed such that a battery is necessary and they can behave erratically without one.
You may want to look into designing your own MPPT controller using a voltage monitor, current monitor, uC, and a variable output switching DC/DC converter. The uC monitors the panel voltage and current and adjusts the DC/DC converter's output voltage (and therefore it's input impedance, since you're driving essentially a fixed resistance) to keep the panel at it's MPP. This won't be an off the shelf solution, you'll need to do some board design and embedded programming.