Speed sensor problems are common and do cause the braking effect you describe under 5mph. If the sensors give faulty readings above that speed they will be disreguarded and the system shut down with the light turned on.
I would suggest that you find some one to replace the sensors for you. The current MB list price is $154 each. I wouldn't expect a quality shop to install parts that you bought elsewhere, but installation isn't that difficult. I have no idea how those sensors could cost over $800.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician