Help with diagnostic codes

I recently bought a 2003 jeep cherokee sport 2.4 petrol, It was perfect on test drive and when we looked at it but on the way home with it the ABS light, Brake light then engine management light all came on, it is also dropping revs and cutting out sometimes when stopping at junctions? My garage plugged it in to their diagnostics and it came up with 2 codes, P0500 Vehicle speed sensor "A" and 17 Rear speed sensor signal failure. Can anyone tell me what they mean and what parts I would need to get? Thank you

Help with diagnostic codes