Unless you are made of money I would have your regular tech diagnose the problem. If you don't have a regular tech, find one!
I seriously doubt that your problems will be cured by a valve job. It sounds to me like a mixture problem. I can see having one maybe two valve guides in severe failure but not all cylinders. Also if you have a uniform oil consumption (equal carbon on old plugs) I would replace the seals first, about a $200 job. The only reason to do the valve job is if you have a compression problem or a loose guide. We have resealed hundreds of 103 motors with complete success. We have done about three valve jobs, all for lack of compression and all with over 200K miles.
Bosch Master, ASE Master, L1
33 years MB technician