Rust is not so nice to K/KE jetronic fuel systems. Check inside the filler neck of the gas tank. Usually rust appears in that area if at all.
The 103 motors all use KE jetronic. They do not have frequency valves. The OVP relay powers the engine controller (and ABS). With it dead numerous symptoms can occur, such as: hard cold starting, hesitations, fluctuating idle, low idle etc.
The O2 sensor also can cause numerous problems and if its more than 60k since replacement it might be a good part to change without testing. Proper testing is the answer for all these things.
Bosch Master, ASE Master, L1
33 years MB technician