Complex sentence: The car is slated to replace the still marketted Proton Saga, which has long based itself on a 1983 Mitsubishi Lancer Fiore platform from the 1980s, and is expected to be the cheapest Proton in the future line of cars offered by the company.
Split into simpler sentences: The car is slated to serve as the next generation Proton Saga, which original series has long based itself on a 1983 Mitsubishi Lancer Fiore platform from the 1980s. The BLM is expected to be the cheapest Proton in the future line of cars offered by the company.