The ST170 is riddled with common problems and Lambda fail is one of them. I am always of the mind set that if i need to change a sensor, or when servicing my ST170 i only buy the best and usually from ford. Nothing wrong with the Bosch Lambda but i would not hunt for a cheaper alternative.
The joy will be trying to remove the old ones. You will most probably need allot of heat, so taking one out will most probably destroy it so not worth cleaning it. If your IMCR has not been sorted do that first
